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This patch prevents the BackForwardCache from caching pages that want to
play media.

My understanding is that MediaWebContentsObserver has an indexed map of
RenderFrameHost to MediaSessionController via its delegate member
MediaSessionControllerManager.

These get added in MediaWebContentsObserver::OnMediaPlaying and sometimes
removed by observing WebContents::RenderFrameHostDeleted.

With the BackForwardCache we do not get this last event and are left in
a non-valid state, so we proactively disable caching for pages involved
with media.

More details on why we sometimes need to disable bfcache:
https://docs.google.com/document/d/1NjZeusdS1kyEkZyfLggndU1A6qVt0Y1sa-LRUxnMoK8

Change-Id: Ibaa3040c9118a8a8dc5f1ad0f0f8d2dbd4609cd7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1832208
Commit-Queue: Arthur Hemery <ahemery@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704165}