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The WebVR frame lifecycle transition from "animating" to "processing" was
confusing due to use of deferred SubmitFrame. Add a new ProcessWebVrCall at
the end of SubmitFrame that can be deferred if needed. A frame counts as
"animating" until that executes.

Refactor VSync logic. WebVrCanAnimateFrame checks if all necessary resources
are available. All the places that provide resources call
WebVrTryStartAnimatingFrame which runs SendVSyncNow if CanAnimateFrame returns
true.

Resources include pending_vsync_ and get_vsync_callback_ along with the
ShouldSkipVSync heuristics and the ui_->CanSendWebVrVSync() check. It should be
generally equivalent, but with reduced wait time if not VSync aligned since it
now reschedules after webvr_deferred_start_processing_ becomes false instead of
waiting for the next OnVSync.

Change-Id: I32f920d3147b0032037b03f72b18df6cf7195751
Reviewed-on: https://chromium-review.googlesource.com/985652
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Reviewed-by: Bill Orr <billorr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547821}