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This CL modifies BeginFrameSourceWebView::BeginFrameObserver to prevent
reentrancy of OnBeginFrame. If the clients of this BeginFrameSource add
or remove observers during OnBeginFrame this will cause the source to
subscribe/unsubscribe from upstream BeginFrameSource and potentially
get missed BeginFrame. To avoid this we save last_used_begin_frames_
before actually processing BeginFrame.

Bug: 1044340
Change-Id: I6df71952c0efa1e8bf7db4953453684b955c42e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2013808Reviewed-by: Bo <boliu@chromium.org>
Commit-Queue: Vasiliy Telezhnikov <vasilyt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734160}