Fix win64 compile failure.

src/global-handles.h

 // Copyright 2007-2008 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 34 matching lines @@
 class ObjectGroup {
  public:
   static ObjectGroup* New(Object*** handles,
                           size_t length,
                           v8::RetainedObjectInfo* info) {
     ASSERT(length > 0);
     ObjectGroup* group = reinterpret_cast<ObjectGroup*>(
         malloc(OFFSET_OF(ObjectGroup, objects_[length])));
     group->length_ = length;
     group->info_ = info;
-    CopyWords(group->objects_, handles, length);
+    CopyWords(group->objects_, handles, static_cast<int>(length));
     return group;
   }
 
   void Dispose() {
     free(this);
   }
 
   size_t length_;
   v8::RetainedObjectInfo* info_;
   Object** objects_[1];  // Variable sized array.
@@ skipping 12 matching lines @@
 class ImplicitRefGroup {
  public:
   static ImplicitRefGroup* New(HeapObject** parent,
                                Object*** children,
                                size_t length) {
     ASSERT(length > 0);
     ImplicitRefGroup* group = reinterpret_cast<ImplicitRefGroup*>(
         malloc(OFFSET_OF(ImplicitRefGroup, children_[length])));
     group->parent_ = parent;
     group->length_ = length;
-    CopyWords(group->children_, children, length);
+    CopyWords(group->children_, children, static_cast<int>(length));
     return group;
   }
 
   void Dispose() {
     free(this);
   }
 
   HeapObject** parent_;
   size_t length_;
   Object** children_[1];  // Variable sized array.
@@ skipping 160 matching lines @@
 
   friend class Isolate;
 
   DISALLOW_COPY_AND_ASSIGN(GlobalHandles);
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_GLOBAL_HANDLES_H_