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Currently, when we enter an immersive XRSession, window.rAF calls stops,
which means we no longer deliver video.requestVideoFrameCallback calls.

This CL adds video.rVFC support to immersive XRSessions. If we find an
immersive XRSession, we schedule video.rVFC callbacks there instead of
the ScriptedAnimationController (SAC).

There can only be one immersive XRSession. If we don't find a session,
but we find that the XRFrameProvider exists, we request to be notified
when an immersive session start. This allows us to keep callbacks going
if the callbacks were already scheduled in the SAC when starting an
XRSession. Simmilarly, if the session ends when callbacks were pending
in the XRSession, we reschedule ourselves in the SAC.

Bug: 1107578
Change-Id: If0acf9445bb789bc848e0043908fc9b769cc5bdd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2406522
Commit-Queue: Thomas Guilbert <tguilbert@chromium.org>
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807653}