Refactor Overlay Pixels

[afterxleep]

Task/Issue URL: https://app.asana.com/0/1204099484721401/1208674332470432/f
Tech Design URL:
CC:

Description:
Refactors Overlay Pixels to use WebKit KVO and backForward lists for improved reliability

Steps to test this PR:

1. Filter the console messages for "PixelKit."
2. Go to Youtube.com
3. Search for MKBHD
4. Select a video
5. Hit Back
6. Confirm m_mac_duckplayer_youtube_overlay_navigation_back pixel fires
7. Select another video
8. Hit Refresh
9. Confirm m_mac_duckplayer_youtube_overlay_navigation_refreshpixel fires
10. Navigate to other Youtube video suggestion
11. Confirm m_mac_duckplayer_youtube_overlay_navigation_within-youtube
12. Visit another website (i.e. example.com)
13. Confirm m_mac_duckplayer_youtube_overlay_navigation_outside-youtube fires

Smoke test going to/from videos and confirm pixels fire as expected.

Demo:

[ayoy]

LGTM, I've tested this extensively and correct pixels are being fired consistently. Leaving minor comments but I believe it's good to go as is.