[wrapper-tracing] Support for incrementally tracing ScopedPersistent

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

Index: third_party/WebKit/Source/platform/heap/WrapperVisitor.h
diff --git a/third_party/WebKit/Source/platform/heap/WrapperVisitor.h b/third_party/WebKit/Source/platform/heap/WrapperVisitor.h
index dc9c5da0851e71d5f0f48a00bf13bcc3dde373be..f9c97d867a0492f1dc48d78bf1b28906e4a5274c 100644
--- a/third_party/WebKit/Source/platform/heap/WrapperVisitor.h
+++ b/third_party/WebKit/Source/platform/heap/WrapperVisitor.h
@@ -23,7 +23,7 @@ template <typename T>
 class Member;
 class ScriptWrappable;
 template <typename T>
-class ScopedPersistent;
+class TraceWrapperV8Reference;
 class TraceWrapperBase;
 template <typename T>
 class TraceWrapperMember;
@@ -94,6 +94,9 @@ class PLATFORM_EXPORT WrapperVisitor {
   template <typename T>
   void traceWrappers(const T* traceable) const {

[haraken, 2016/11/03 14:25:04]

Would you help me understand who uses this method? I'm afraid that this method
can be used for 'catch-all' of traceWrappers().

[Michael Lippautz, 2016/11/03 14:53:41]

Yes, one has to be careful with traceWrappers overloads. It's used for anything
that inherits from TraceWrapperBase. Once we are in the same component we can
improve this signature. 

     static_assert(sizeof(T), "T must be fully defined");
+    // Ideally, we'd assert that we can cast to TraceWrapperBase here.
+    static_assert(IsGarbageCollectedType<T>::value,
+                  "only garbage collected objects can be");

[haraken, 2016/11/03 14:25:04]

Only garbage collected objects can be used in traceWrappers().

[Michael Lippautz, 2016/11/03 14:53:41]

Done.

 
     if (!traceable) {
       return;
@@ -136,11 +139,8 @@ class PLATFORM_EXPORT WrapperVisitor {
   }
 
   virtual void traceWrappers(
-      const ScopedPersistent<v8::Value>* persistent) const = 0;
-  virtual void traceWrappers(
-      const ScopedPersistent<v8::Object>* persistent) const = 0;
-  virtual void markWrapper(
-      const v8::PersistentBase<v8::Object>* persistent) const = 0;
+      const TraceWrapperV8Reference<v8::Value>*) const = 0;

[haraken, 2016/11/03 14:25:04]

It's inconsistent that traceWrappers(const TraceWrapperMember<T>& t) takes a
reference but traceWrappers(const TraceWrapperV8Reference<v8::Value>*) takes a
pointer.

[Michael Lippautz, 2016/11/03 14:53:41]

Done.

+  virtual void markWrapper(const v8::PersistentBase<v8::Value>*) const = 0;
 
   virtual void dispatchTraceWrappers(const TraceWrapperBase*) const = 0;
 #define DECLARE_DISPATCH_TRACE_WRAPPERS(ClassName) \