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Observers can be added while calling OnBeginFrame on existing observers.
In these cases, we reentrantly take ScopedOnBeginFrame.

There was a DCHECK preventing this, but given that this is supported
(we create a the ObserverList with iteration policy EXISTING_ONLY to
account for this), we should handle this case.

This allows for reentrant scopes in certain cases, making them a no-op.

Change-Id: Id5efacf89250fb8d93fe0f6bcb2ce626c1831b2b
Reviewed-on: https://chromium-review.googlesource.com/c/1354136Reviewed-by: Bo <boliu@chromium.org>
Commit-Queue: Eric Karl <ericrk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613234}