Fix/implement new write barrier for x64.

src/serialize.cc

Index: src/serialize.cc
===================================================================
--- src/serialize.cc	(revision 6184)
+++ src/serialize.cc	(working copy)
@@ -652,6 +652,8 @@
 
 void Deserializer::Deserialize() {
   // Don't GC while deserializing - just expand the heap.
+  Address* write_buffer_top =
+      reinterpret_cast<Address*>(Heap::write_buffer_top());
   AlwaysAllocateScope always_allocate;
   // Don't use the free lists while deserializing.
   LinearAllocationScope allocate_linearly;
@@ -668,6 +670,7 @@
   Heap::IterateWeakRoots(this, VISIT_ALL);
 
   Heap::set_global_contexts_list(Heap::undefined_value());
+  Heap::public_set_write_buffer_top(write_buffer_top);
 }