Prevent delegate access after player removal has been signaled.

Should result in nullptr crashes which may be easier to debug to
find culprits which are violating this.

R=tguilbert

Bug: 1035556
Change-Id: I14fe4ad3680e536b1193b47d0127ba46cda6b93e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2451383
Commit-Queue: Thomas Guilbert <tguilbert@chromium.org>
Auto-Submit: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813993}

media/blink/webmediaplayer_impl.cc

@@ -485,6 +485,7 @@ WebMediaPlayerImpl::~WebMediaPlayerImpl() {
 
   delegate_->PlayerGone(delegate_id_);
   delegate_->RemoveObserver(delegate_id_);
+  delegate_ = nullptr;
 
   // Finalize any watch time metrics before destroying the pipeline.
   watch_time_reporter_.reset();

media/blink/webmediaplayer_impl.h

@@ -748,7 +748,7 @@ class MEDIA_BLINK_EXPORT WebMediaPlayerImpl
   // Document::shutdown() is called before the frame detaches (and before the
   // frame is destroyed). RenderFrameImpl owns |delegate_| and is guaranteed
   // to outlive |this|; thus it is safe to store |delegate_| as a raw pointer.
-  blink::WebMediaPlayerDelegate* const delegate_;
+  blink::WebMediaPlayerDelegate* delegate_;
   int delegate_id_ = 0;
 
   // The playback state last reported to |delegate_|, to avoid setting duplicate