Revert of [oilpan]: Remove support for tracing off-heap hashmaps.

Source/core/dom/ExecutionContext.h

Index: Source/core/dom/ExecutionContext.h
diff --git a/Source/core/dom/ExecutionContext.h b/Source/core/dom/ExecutionContext.h
index ccb887733cefe46d789cb35d8b1f7b96b33f70c9..b5fe83717fa75deab0076a8f4383ae0d5800d185 100644
--- a/Source/core/dom/ExecutionContext.h
+++ b/Source/core/dom/ExecutionContext.h
@@ -56,10 +56,11 @@
 class ScriptCallStack;
 
 class ExecutionContext
-    : public LifecycleContext<ExecutionContext>
-    , public WillBeHeapSupplementable<ExecutionContext> {
+    : public WillBeGarbageCollectedMixin
+    , public LifecycleContext<ExecutionContext>
+    , public Supplementable<ExecutionContext> {
 public:
-    virtual void trace(Visitor*) OVERRIDE;
+    virtual void trace(Visitor*);
 
     // Delegating to ExecutionContextClient
     bool isDocument() const { return m_client && m_client->isDocument(); }