Move the Widget hierarchy to the Oilpan heap.

Source/platform/Widget.h

Index: Source/platform/Widget.h
diff --git a/Source/platform/Widget.h b/Source/platform/Widget.h
index ffe2ac6e1dc1327645a8548d5c46e4b6553e23c6..9c375b4723e4326fca4910ef659f5524dd1cb2bc 100644
--- a/Source/platform/Widget.h
+++ b/Source/platform/Widget.h
@@ -31,6 +31,7 @@
 #include "platform/PlatformExport.h"
 #include "platform/geometry/FloatPoint.h"
 #include "platform/geometry/IntRect.h"
+#include "platform/heap/Handle.h"
 #include "wtf/Forward.h"
 #include "wtf/RefCounted.h"
 
@@ -48,7 +49,7 @@ class HostWindow;
 // Widgets are connected in a hierarchy, with the restriction that plugins and
 // scrollbars are always leaves of the tree. Only ScrollViews can have children
 // (and therefore the Widget class has no concept of children).
-class PLATFORM_EXPORT Widget : public RefCounted<Widget> {
+class PLATFORM_EXPORT Widget : public RefCountedWillBeGarbageCollectedFinalized<Widget> {
 public:
     Widget();
     virtual ~Widget();
@@ -131,8 +132,10 @@ public:
     virtual void detach() { }

[haraken, 2014/09/29 14:16:36]

When is Widget::detach() called?

It's a bit confusing that the Widget hierarchy has both detach() and dispose()
(PluginView has dispose()).

[sof, 2014/10/02 14:03:54]

It's called as part of detaching HTMLPlugInElement, to detach plugin containers
only. 

But with the separate dispose() added for PluginView, which seems unavoidable,
detach() isn't providing anything over dispose(). So we can retire that
addition; nice! Now done.

 #endif
 
+    virtual void trace(Visitor*);
+
 private:
-    Widget* m_parent;
+    RawPtrWillBeMember<Widget> m_parent;
     IntRect m_frame;
     bool m_selfVisible;
     bool m_parentVisible;