Refactor the Heap into ThreadHeap to prepare for per thread heaps

third_party/WebKit/Source/platform/heap/PersistentNode.h

Index: third_party/WebKit/Source/platform/heap/PersistentNode.h
diff --git a/third_party/WebKit/Source/platform/heap/PersistentNode.h b/third_party/WebKit/Source/platform/heap/PersistentNode.h
index 62d176d18f1ff5cc9fd0b762e6645167223de631..42147dd3a9e0818aba90d12307880d5a4fa80837 100644
--- a/third_party/WebKit/Source/platform/heap/PersistentNode.h
+++ b/third_party/WebKit/Source/platform/heap/PersistentNode.h
@@ -149,7 +149,8 @@ public:
         --m_persistentCount;
 #endif
     }
-    void tracePersistentNodes(Visitor*);
+    using ShouldTraceCallback = bool (*)(Visitor*, PersistentNode*);
+    void tracePersistentNodes(Visitor*, ShouldTraceCallback = nullptr);
     int numberOfPersistents();
 
 private:
@@ -184,11 +185,13 @@ public:
     void tracePersistentNodes(Visitor* visitor)
     {
         MutexLocker lock(m_mutex);
-        m_persistentRegion->tracePersistentNodes(visitor);
+        m_persistentRegion->tracePersistentNodes(visitor, CrossThreadPersistentRegion::shouldTracePersistentNode);
     }
 
     void prepareForThreadStateTermination(ThreadState*);
 
+    static bool shouldTracePersistentNode(Visitor*, PersistentNode*);
+
 private:
     // We don't make CrossThreadPersistentRegion inherit from PersistentRegion
     // because we don't want to virtualize performance-sensitive methods