Additional minor cleanup regarding CallWrapper: Use the null object pattern.

src/x64/macro-assembler-x64.h

Index: src/x64/macro-assembler-x64.h
===================================================================
--- src/x64/macro-assembler-x64.h	(revision 7766)
+++ src/x64/macro-assembler-x64.h	(working copy)
@@ -244,32 +244,32 @@
                   const ParameterCount& expected,
                   const ParameterCount& actual,
                   InvokeFlag flag,
-                  CallWrapper* call_wrapper = NULL);
+                  const CallWrapper& call_wrapper = NullCallWrapper());
 
   void InvokeCode(Handle<Code> code,
                   const ParameterCount& expected,
                   const ParameterCount& actual,
                   RelocInfo::Mode rmode,
                   InvokeFlag flag,
-                  CallWrapper* call_wrapper = NULL);
+                  const CallWrapper& call_wrapper = NullCallWrapper());
 
   // Invoke the JavaScript function in the given register. Changes the
   // current context to the context in the function before invoking.
   void InvokeFunction(Register function,
                       const ParameterCount& actual,
                       InvokeFlag flag,
-                      CallWrapper* call_wrapper = NULL);
+                      const CallWrapper& call_wrapper = NullCallWrapper());
 
   void InvokeFunction(JSFunction* function,
                       const ParameterCount& actual,
                       InvokeFlag flag,
-                      CallWrapper* call_wrapper = NULL);
+                      const CallWrapper& call_wrapper = NullCallWrapper());
 
   // Invoke specified builtin JavaScript function. Adds an entry to
   // the unresolved list if the name does not resolve.
   void InvokeBuiltin(Builtins::JavaScript id,
                      InvokeFlag flag,
-                     CallWrapper* call_wrapper = NULL);
+                     const CallWrapper& call_wrapper = NullCallWrapper());
 
   // Store the function for the given builtin in the target register.
   void GetBuiltinFunction(Register target, Builtins::JavaScript id);
@@ -1126,7 +1126,7 @@
                       Register code_register,
                       LabelType* done,
                       InvokeFlag flag,
-                      CallWrapper* call_wrapper);
+                      const CallWrapper& call_wrapper);
 
   // Activation support.
   void EnterFrame(StackFrame::Type type);
@@ -1936,7 +1936,7 @@
                                     Register code_register,
                                     LabelType* done,
                                     InvokeFlag flag,
-                                    CallWrapper* call_wrapper) {
+                                    const CallWrapper& call_wrapper) {
   bool definitely_matches = false;
   NearLabel invoke;
   if (expected.is_immediate()) {
@@ -1985,9 +1985,9 @@
     }
 
     if (flag == CALL_FUNCTION) {
-      if (call_wrapper != NULL) call_wrapper->BeforeCall(CallSize(adaptor));
+      call_wrapper.BeforeCall(CallSize(adaptor));
       Call(adaptor, RelocInfo::CODE_TARGET);
-      if (call_wrapper != NULL) call_wrapper->AfterCall();
+      call_wrapper.AfterCall();
       jmp(done);
     } else {
       Jump(adaptor, RelocInfo::CODE_TARGET);