Bug 1808155 Part 2: Make VIDEO_LOG note events that will cause the video layer...

Bug 1808155 Part 2: Make VIDEO_LOG note events that will cause the video layer to rebuild. r=mstange

There are only a few things that could cause the video layer to be
rebuilt:
1) The mMutatedSpecializeVideo flag being set.
2) A new layer object being created to host for the video.

This patch more thoroughly documents instances of case 1 by adding
VIDEO_LOG message to anything that sets the mMutatedSpecializeVideo flag.

It also makes some attempt to identify case 2 by adding a VIDEO_LOG
message whenever we destroy a layer that has ever displayed a video
texture. If a log message like this is followed by a message that a new
video layer is created, it is a strong signal that we are unexpectedly
throwing away our external surface handles when we'd like to keep using
them.

Differential Revision: https://phabricator.services.mozilla.com/D171301