Introduce infrastructure for tracing ScriptWrappables.

third_party/WebKit/Source/bindings/core/v8/DOMWrapperWorld.h

Index: third_party/WebKit/Source/bindings/core/v8/DOMWrapperWorld.h
diff --git a/third_party/WebKit/Source/bindings/core/v8/DOMWrapperWorld.h b/third_party/WebKit/Source/bindings/core/v8/DOMWrapperWorld.h
index c782ba01832de8633088fd1fa468b2a1692d5840..3a2ade64f2f2d3598cf731de6454664d72aa567f 100644
--- a/third_party/WebKit/Source/bindings/core/v8/DOMWrapperWorld.h
+++ b/third_party/WebKit/Source/bindings/core/v8/DOMWrapperWorld.h
@@ -54,6 +54,7 @@ enum WorldIdConstants {
 };
 
 class DOMObjectHolderBase;
+class DOMWrapperWorldVisitor;
 template<typename T> class DOMObjectHolder;
 
 // This class represent a collection of DOM wrappers for a specific world.
@@ -69,6 +70,7 @@ public:
 
     static bool isolatedWorldsExist() { return isolatedWorldCount; }
     static void allWorldsInMainThread(Vector<RefPtr<DOMWrapperWorld>>& worlds);
+    static void visitAllWorldsInMainThread(DOMWrapperWorldVisitor&);
 
     static DOMWrapperWorld& world(v8::Local<v8::Context> context)
     {
@@ -145,6 +147,12 @@ private:
     HashSet<OwnPtr<DOMObjectHolderBase>> m_domObjectHolders;
 };
 
+class DOMWrapperWorldVisitor {
+STACK_ALLOCATED();
+public:
+    virtual void visit(DOMWrapperWorld&) = 0;
+};
+
 } // namespace blink
 
 #endif // DOMWrapperWorld_h