Fix/implement new write barrier for x64.

src/x64/code-stubs-x64.h

Index: src/x64/code-stubs-x64.h
===================================================================
--- src/x64/code-stubs-x64.h	(revision 6183)
+++ src/x64/code-stubs-x64.h	(working copy)
@@ -63,6 +63,21 @@
 };
 
 
+class WriteBufferOverflowStub: public CodeStub {
+ public:
+  explicit WriteBufferOverflowStub(SaveFPRegsMode save_fp)
+      : save_doubles_(save_fp) { }
+
+  void Generate(MacroAssembler* masm);
+
+ private:
+  SaveFPRegsMode save_doubles_;
+
+  Major MajorKey() { return WriteBufferOverflow; }
+  int MinorKey() { return (save_doubles_ == kSaveFPRegs) ? 1 : 0; }
+};
+
+
 // Flag that indicates how to generate code for the stub GenericBinaryOpStub.
 enum GenericBinaryFlags {
   NO_GENERIC_BINARY_FLAGS = 0,