Make setMicrophoneActive and setCameraActive return promises.

Define steps for each of these methods.
Mention the possibility to mute/unmute tracks based on setMicrophoneActive/setCameraActive calls.
Mention the possibility for the user agent to deny the mutation of capture states via setMicrophoneActive/setCameraActive calls.

index.bs

@@ -82,6 +82,11 @@ urlPrefix: https://www.w3.org/TR/permissions-policy-1/; spec: PermissionsPolicy
 urlPrefix: https://html.spec.whatwg.org/multipage/dom.html; spec: dom
     type: dfn
         text: permissions policy; url:#concept-document-permissions-policy
+urlPrefix: https://www.w3.org/TR/mediacapture-streams/; spec: mediacapture-main
+    type: dfn
+        text: MediaStreamTrack; url:#mediastreamtrack
+        text: MediaStreamTrack muted state; url:#track-muted
+        text: set MediaStreamTrack muted state; url:#set-track-muted
 </pre>
 
 <h2 id="introduction">Introduction</h2>
@@ -776,9 +781,9 @@ interface MediaSession {
 
   undefined setPositionState(optional MediaPositionState state = {});
 
-  undefined setMicrophoneActive(boolean active);
+  Promise&lt;undefined&gt; setMicrophoneActive(boolean active);
 
-  undefined setCameraActive(boolean active);
+  Promise&lt;undefined&gt; setCameraActive(boolean active);
 };
 </pre>
 
@@ -912,16 +917,78 @@ interface MediaSession {
     </li>
   </ul>
 </p>
-
 <p>
-  The <dfn method for=MediaSession>setMicrophoneActive(active)</dfn> and
-  <dfn method for=MediaSession>setCameraActive(active)</dfn> methods indicate to
-  the user agent whether the microphone and camera are currently considered by
-  the page to be active (e.g. if the microphone is considered "muted" by the
-  page since it is no longer sending audio through to a call, then the page can
-  invoke <code>setMicrophoneActive(false)</code>).
-  It is RECOMMENDED that the user agent respect the microphone and camera
-  states indicated by the page in this UI.
+  The <dfn method for=MediaSession>setMicrophoneActive(active)</dfn> method
+  indicates to the user agent whether the microphone is considered by the page
+  to be active (e.g. if the microphone is considered "muted" by the page since
+  it is no longer sending audio through a call, then the page can invoke
+  <code>setMicrophoneActive(false)</code>). When invoked, it MUST perform the
+  following steps:
+  <ul>
+    <li>
+      Let <var>p</var> be a new promise.
+    </li>
+    <li>
+      <a>Queue a task</a> to run the following substeps:
+      <ul>
+        <li>The user agent MAY wait to proceed, for instance if
+        <var>active</var> is <code>true</code> and the relevant global object's
+        associated <code>Document</code> visiblity state is not "visible".</li>
+        <li>If the user agent denies permission to change the microphone state
+        to <var>active</var>, reject <var>p</var> with a <a
+        exception>NotAllowedError</a>.</li>
+        <li>If the user agent exposes the microphone state via
+        <a>MediaStreamTrack muted state</a>, the user agent MUST <a>queue a
+        task</a> to <a>set MediaStreamTrack muted state</a> on any affected
+        <a>MediaStreamTrack</a>.</li>
+        <li>Update the user agent microphone state UI according
+        <var>active</var>.</li>
+        <li>Resolve <var>p</var> with <code>undefined</code>.</li>
+      </ul>
+    </li>
+    <li>
+      Return <var>p</var> be a new promise.
+    </li>
+  </ul>
+</p>
+<p>
+  Similarly, the <dfn method for=MediaSession>setCameraActive(active)</dfn>
+  method indicates to the user agent whether the camera is considered by the
+  page to be active. When invoked, it MUST perform the following steps:
+  <ul>
+    <li>
+      Let <var>p</var> be a new promise.
+    </li>
+    <li>
+      <a>Queue a task</a> to run the following substeps:
+      <ul>
+        <li>The user agent MAY wait to proceed, for instance if
+        <var>active</var> is <code>true</code> and the relevant global object's
+        associated <code>Document</code> visiblity state is not "visible".</li>
+        <li>If the user agent denies permission to change the camera state to
+        <var>active</var>, reject <var>p</var> with a <a
+        exception>NotAllowedError</a>.</li>
+        <li>If the user agent exposes the camera state via <a>MediaStreamTrack
+        muted state</a>, the user agent MUST <a>queue a task</a> to <a>set
+        MediaStreamTrack muted state</a> on any affected
+        <a>MediaStreamTrack</a>.</li>
+        <li>Update the user agent camera state UI according
+        <var>active</var>.</li>
+        <li>Resolve <var>p</var> with <code>undefined</code>.</li>
+      </ul>
+    </li>
+    <li>
+      Return <var>p</var> be a new promise.
+    </li>
+  </ul>
+</p>
+<p class=note>
+  Both the <a>setMicrophoneActive(active)</a> and <a>setCameraActive(active)</a>
+  methods may reject based on user agent specific heuristics. This might in
+  particular happen when the web page asks to activate (aka unmute) microphone
+  or camera. The user agent may decide to require transient activation in that
+  case. It may also require user input through a prompt to make the actual
+  decision.
 </p>
 
 <p>