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This happens synchronously with the destruction of WebMediaPlayerImpl
so that the devtools backend for MediaLog knows it shouldn't be sending
events since the page may have been destroyed. Events that happen
after WEBMEDIAPLAYER_DESTROYED get dropped from _only_ the devtools
version.

This should fix the nullptr deref bug caused by the destruction order
of the blink objects and the WebMediaPlayerImpl::DestructionHelper.

Bug: 794255
Bug: 1014433
Change-Id: I25e2110a31561367baf7104c77a8e5301b5afb0c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1891078
Commit-Queue: Ted Meyer <tmathmeyer@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711858}