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Previously, we would use one weak pointer for the media
WebContentsObserver for all power experiment callbacks.  However,
since the experiment manager posts callbacks, it's possible that a
callback will run after its RenderFrameHost is deleted.  Since we
use the RenderFrameHost to notify the renderer, this would be a
problem in that case.

This CL adds per-frame weak pointers that are invalidated after the
frame is destroyed, so that any pending callbacks don't run.

Change-Id: I02835ebfd620816e385fd215472dd78ecbeae4fa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1897701Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Commit-Queue: Frank Liberato <liberato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712651}