Oilpan: handle MediaControls like the GCed object it is.

Source/core/html/shadow/MediaControlElementTypes.cpp

Index: Source/core/html/shadow/MediaControlElementTypes.cpp
diff --git a/Source/core/html/shadow/MediaControlElementTypes.cpp b/Source/core/html/shadow/MediaControlElementTypes.cpp
index 387020444635f3e000ee38183c073b1a765d4819..0c8d6f3e9d5fcb643dcf25905c34ec18735d7366 100644
--- a/Source/core/html/shadow/MediaControlElementTypes.cpp
+++ b/Source/core/html/shadow/MediaControlElementTypes.cpp
@@ -68,7 +68,7 @@ MediaControlElementType mediaControlElementType(Node* node)
 }
 
 MediaControlElement::MediaControlElement(MediaControls& mediaControls, MediaControlElementType displayType, HTMLElement* element)
-    : m_mediaControls(mediaControls)
+    : m_mediaControls(&mediaControls)
     , m_displayType(displayType)
     , m_element(element)
     , m_isWanted(true)
@@ -118,6 +118,7 @@ void MediaControlElement::setDisplayType(MediaControlElementType displayType)
 
 DEFINE_TRACE(MediaControlElement)
 {
+    visitor->trace(m_mediaControls);
     visitor->trace(m_element);
 }