Decouple root visitors from object visitors.

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index 52cb49baa6ecaa3262574f2db24a22fd8fae92da..033ca1019168d8c1c1aab49253497b52d4a89955 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -78,6 +78,7 @@ class Logger;
 class MaterializedObjectStore;
 class OptimizingCompileDispatcher;
 class RegExpStack;
+class RootVisitor;
 class RuntimeProfiler;
 class SaveContext;
 class SetupIsolateDelegate;
@@ -815,9 +816,9 @@ class Isolate {
   void InvokeApiInterruptCallbacks();
 
   // Administration
-  void Iterate(ObjectVisitor* v);
-  void Iterate(ObjectVisitor* v, ThreadLocalTop* t);
-  char* Iterate(ObjectVisitor* v, char* t);
+  void Iterate(RootVisitor* v);
+  void Iterate(RootVisitor* v, ThreadLocalTop* t);
+  char* Iterate(RootVisitor* v, char* t);
   void IterateThread(ThreadVisitor* v, char* t);
 
   // Returns the current native context.
@@ -1077,7 +1078,7 @@ class Isolate {
 
   AccessCompilerData* access_compiler_data() { return access_compiler_data_; }
 
-  void IterateDeferredHandles(ObjectVisitor* visitor);
+  void IterateDeferredHandles(RootVisitor* visitor);
   void LinkDeferredHandles(DeferredHandles* deferred_handles);
   void UnlinkDeferredHandles(DeferredHandles* deferred_handles);