X64 implementation: Add arguments object to context when needed.

src/x64/codegen-x64.h

Index: src/x64/codegen-x64.h
===================================================================
--- src/x64/codegen-x64.h	(revision 2277)
+++ src/x64/codegen-x64.h	(working copy)
@@ -273,8 +273,16 @@
 };
 
 
+// -------------------------------------------------------------------------
+// Arguments allocation mode
 
+enum ArgumentsAllocationMode {
+  NO_ARGUMENTS_ALLOCATION,
+  EAGER_ARGUMENTS_ALLOCATION,
+  LAZY_ARGUMENTS_ALLOCATION
+};
 
+
 // -------------------------------------------------------------------------
 // CodeGenerator
 
@@ -383,6 +391,12 @@
   // target (which can not be done more than once).
   void GenerateReturnSequence(Result* return_value);
 
+  // Returns the arguments allocation mode.
+  ArgumentsAllocationMode ArgumentsMode() const;
+
+  // Store the arguments object and allocate it if necessary.
+  Result StoreArgumentsObject(bool initial);
+
   // The following are used by class Reference.
   void LoadReference(Reference* ref);
   void UnloadReference(Reference* ref);