Bug 1737682 Part 1: Log whether or not we are hitting the detached state. r=mstange

Bug 1737682 Part 1: Log whether or not we are hitting the detached state. r=mstange

There are 10 enum cases that we track internally, one of which isn't
emitted as telemetry. These cases are 0-indexed to match the enum values.

0 / NotVideo: Not used for telemetry. No video is showing.

1 / Success: We are showing exactly one video and we believe we are
hitting the video low power mode.

2 / FailMultipleVideo: There is more than one video visible.

3 / FailWindowed: The video is being viewed in windowed mode, not
fullscreen mode, so low power mode is not possible.

4 / FailOverlaid: The video has something on top of it (like captions).

5 / FailBacking: The layer directly underneath the video does not cover
the window or does not have a black background.

6 / FailMacOSVersion: The system is running a too-early version of macOS.

7 / FailPref: The user has disabled the
`gfx.core-animation.specialize-video` pref.

8 / FailSurface: The video is encoded in such a way we can't decode it to
a qualifying pixel format.

9 / FailEnqueue: The video didn't enqueue properly, and we fell back to a
non-video display path.

Differential Revision: https://phabricator.services.mozilla.com/D129453