Load callback data from weak cell instead of embedding it in handler.

src/x64/macro-assembler-x64.h

Index: src/x64/macro-assembler-x64.h
diff --git a/src/x64/macro-assembler-x64.h b/src/x64/macro-assembler-x64.h
index 2308d290ac72542edcf32c1a03a02e09f53bec4c..9f25d60ddfbfd4b00b1fd6dabbea45d32d9c8e31 100644
--- a/src/x64/macro-assembler-x64.h
+++ b/src/x64/macro-assembler-x64.h
@@ -846,6 +846,8 @@ class MacroAssembler: public Assembler {
   // Compare the given value and the value of weak cell.
   void CmpWeakValue(Register value, Handle<WeakCell> cell, Register scratch);
 
+  void GetWeakValue(Register value, Handle<WeakCell> cell);
+
   // Load the value of the weak cell in the value register. Branch to the given
   // miss label if the weak cell was cleared.
   void LoadWeakValue(Register value, Handle<WeakCell> cell, Label* miss);