Oilpan: have MediaController weakly refer to its media elements.

Source/core/html/MediaController.h

Index: Source/core/html/MediaController.h
diff --git a/Source/core/html/MediaController.h b/Source/core/html/MediaController.h
index 29b7d9ac638ed25241d2748f675d07f836f46396..462dad711abdd694c7a32b572f9a46ccbaf5e502 100644
--- a/Source/core/html/MediaController.h
+++ b/Source/core/html/MediaController.h
@@ -29,9 +29,9 @@
 #include "bindings/v8/ScriptWrappable.h"
 #include "core/events/EventTarget.h"
 #include "core/html/HTMLMediaElement.h"
+#include "wtf/LinkedHashSet.h"
 #include "wtf/PassRefPtr.h"
 #include "wtf/RefCounted.h"
-#include "wtf/Vector.h"
 
 namespace WebCore {
 
@@ -86,6 +86,8 @@ public:
 
     void clearExecutionContext() { m_executionContext = 0; }
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     MediaController(ExecutionContext*);
     void reportControllerState();
@@ -106,7 +108,8 @@ private:
 
     friend class HTMLMediaElement;
     friend class MediaControllerEventListener;
-    Vector<HTMLMediaElement*> m_mediaElements;
+    typedef WillBeHeapLinkedHashSet<RawPtrWillBeWeakMember<HTMLMediaElement> > MediaElementSequence;

[acolwell GONE FROM CHROMIUM, 2014/06/07 00:04:50]

I'm a little fuzzy on these Oilpan types. If you use this, does this mean that
media elements can disappear from this list if the JavaScript application
doesn't have any references to the elements and they aren't in the document, but
JavaScript DOES have a reference to this controller? If so, then I think that
might be bad. 

Say I have several audio elements I want to control with the same controller,
but don't really need them in the document. Conceptually I should be able to
just hold onto the controller and still control the media elements eventhough
they have gone out of scope. I think the web developer might be surprised if
these elements disappear.

[haraken, 2014/06/07 03:59:00]

Your understanding is correct.

If you want to keep the media elements alive while the controller is alive, we
can use HeapLinkedHashSet<Member<HTMLMediaElement>>. In fact, we _do_ want to do
that because we don't want to use weak members as much as possible. However, as
far as I understand the current code, the media elements are not kept alive
while the controller is alive, so we used
HeapLinkedHashSet<WeakMember<HTMLMediaElement>> to keep the current behavior.

In short, if you're happy with keeping the media elements alive while the
controller is alive, we're very happy to make the change :)

[sof, 2014/06/07 05:33:30]

Comments #5 and #6 touch on what the spec promises to be the behavior -- we're
not able to locate much on a slaved element's guaranteed lifetime (other than
the lifetime of such an audio element is mentioned elsewhere as needing to stay
around when detached from the document, if still playing.)

Might it be better to wait with doing this change until/if Oilpan is the
default? Otherwise you have behavioral differences across the setting of
enable_oilpan.

+    MediaElementSequence m_mediaElements;
     bool m_paused;
     double m_defaultPlaybackRate;
     double m_volume;