Handle new "tab closed" response from Chrome

[fregante]

• Presumably fixes A listener indicated an asynchronous response by returning true, but the message channel closed before a response was received #384

[fregante]

CI is failing for unrelated reasons

[Rob--W]

Could you add a unit test?

webextension-polyfill/test/test-runtime-onMessage.js

Lines 242 to 265 in 2ece178

	it("resolves to undefined when no listeners reply", () => {
	const fakeChrome = {
	runtime: {
	// This error message is defined as CHROME_SEND_MESSAGE_CALLBACK_NO_RESPONSE_MESSAGE
	// in the polyfill sources and it is used to recognize when Chrome has detected that
	// none of the listeners replied.
	lastError: {
	message: "The message port closed before a response was received.",
	},
	sendMessage: sinon.stub(),
	},
	};
	fakeChrome.runtime.sendMessage.onFirstCall().callsArgWith(1, [undefined]);
	return setupTestDOMWindow(fakeChrome).then(window => {
	const promise = window.browser.runtime.sendMessage("some_message");
	ok(fakeChrome.runtime.sendMessage.calledOnce, "sendMessage has been called once");
	return promise.then(reply => {
	deepEqual(reply, undefined, "sendMessage promise should be resolved to undefined");
	});
	});
	});

If you'd like, it would also be nice to add an integration test that triggers the error in a real Chrome browser (in case the error message changes), at https://github.com/mozilla/webextension-polyfill/tree/master/test/fixtures

[fregante]

Sorry I haven't had time to finish this.