Oilpan: Make MediaControllers ExecutionContext a weak pointer.

Cause of a flaky ASAN use-after-poison in media/video-frame-accurate-seek.html

R=ager@chromium.org, haraken@chromium.org
BUG=

Review URL: https://codereview.chromium.org/333813004

git-svn-id: svn://svn.chromium.org/blink/trunk@176111 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/core/html/HTMLMediaElement.cpp

@@ -3190,8 +3190,12 @@ bool HTMLMediaElement::hasPendingActivity() const
 
 void HTMLMediaElement::contextDestroyed()
 {
+    // With Oilpan the ExecutionContext is weakly referenced from the media
+    // controller and so it will clear itself on destruction.
+#if !ENABLE(OILPAN)
     if (m_mediaController)
         m_mediaController->clearExecutionContext();
+#endif
     ActiveDOMObject::contextDestroyed();
 }
 

third_party/WebKit/Source/core/html/MediaController.cpp

@@ -613,6 +613,7 @@ void MediaController::trace(Visitor* visitor)
 {
     visitor->trace(m_mediaElements);
     visitor->trace(m_pendingEventsQueue);
+    visitor->trace(m_executionContext);
     EventTargetWithInlineData::trace(visitor);
 }
 

third_party/WebKit/Source/core/html/MediaController.h

@@ -84,7 +84,9 @@ public:
     bool isRestrained() const;
     bool isBlocked() const;
 
-    void clearExecutionContext() { m_executionContext = 0; }
+#if !ENABLE(OILPAN)
+    void clearExecutionContext() { m_executionContext = nullptr; }
+#endif
 
     virtual void trace(Visitor*) OVERRIDE;
 
@@ -124,7 +126,7 @@ private:
     OwnPtrWillBeMember<GenericEventQueue> m_pendingEventsQueue;
     mutable Timer<MediaController> m_clearPositionTimer;
     OwnPtr<Clock> m_clock;
-    ExecutionContext* m_executionContext;
+    RawPtrWillBeWeakMember<ExecutionContext> m_executionContext;
     Timer<MediaController> m_timeupdateTimer;
     double m_previousTimeupdateTime;
 };