This patch adds support for the capture and replaying of multiple frames
during normal operation of Firefox. Ctrl + Shift + 6 starts capturing
and pressing it again stops capturing. It attempts to capture the minimum
amount of data required to replay a sequence for debugging purposes.

There are several known limitations, particularly surrounding replaying
when transitioning between snapshots of the resource cache. It will
reload the entire document set, causing greater delay between frames.
Should you advance too quickly, it may also panic due to a race between
the current frame still being generated, and the new frame resetting the
resource cache state. These should be resolved with time, and the
current implementation should be workable to at least capture/debug most
animated issues with some effort.

It also adds support for loading dynamic properties which is necessary
for accurate replaying of a captured frame (sequence or individual)
which are in the middle of an animation.

Differential Revision: https://phabricator.services.mozilla.com/D59755