Fix/implement new write barrier for x64.

src/heap.h

Index: src/heap.h
===================================================================
--- src/heap.h	(revision 6183)
+++ src/heap.h	(working copy)
@@ -48,6 +48,7 @@
   V(Map, two_pointer_filler_map, TwoPointerFillerMap)                          \
   /* Cluster the most popular ones in a few cache lines here at the top.    */ \
   V(Smi, stack_limit, StackLimit)                                              \
+  V(Smi, write_buffer_top, WriteBufferTop)                                     \
   V(Object, undefined_value, UndefinedValue)                                   \
   V(Object, the_hole_value, TheHoleValue)                                      \
   V(Object, null_value, NullValue)                                             \
@@ -905,12 +906,20 @@
     roots_[kEmptyScriptRootIndex] = script;
   }
 
+  static void public_set_write_buffer_top(Address* top) {
+    roots_[kWriteBufferTopRootIndex] = reinterpret_cast<Smi*>(top);
+  }
+
   // Update the next script id.
   static inline void SetLastScriptId(Object* last_script_id);
 
   // Generated code can embed this address to get access to the roots.
   static Object** roots_address() { return roots_; }
 
+  static Address* write_buffer_top_address() {
+    return reinterpret_cast<Address*>(&roots_[kWriteBufferTopRootIndex]);
+  }
+
   // Get address of global contexts list for serialization support.
   static Object** global_contexts_list_address() {
     return &global_contexts_list_;