Distinguish weak references in heap snapshots, group GC roots.

src/bootstrapper.cc

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 281 matching lines @@
   Handle<Map> strict_mode_function_instance_map_writable_prototype_;
   Handle<JSFunction> throw_type_error_function;
 
   BootstrapperActive active_;
   friend class Bootstrapper;
 };
 
 
 void Bootstrapper::Iterate(ObjectVisitor* v) {
   extensions_cache_.Iterate(v);
-  v->Synchronize("Extensions");
+  v->Synchronize(VisitorSynchronization::kExtensions);
 }
 
 
 Handle<Context> Bootstrapper::CreateEnvironment(
     Isolate* isolate,
     Handle<Object> global_object,
     v8::Handle<v8::ObjectTemplate> global_template,
     v8::ExtensionConfiguration* extensions) {
   HandleScope scope;
   Handle<Context> env;
@@ skipping 2042 matching lines @@
   return from + sizeof(NestingCounterType);
 }
 
 
 // Called when the top-level V8 mutex is destroyed.
 void Bootstrapper::FreeThreadResources() {
   ASSERT(!IsActive());
 }
 
 } }  // namespace v8::internal