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

src/assembler.h

 // Copyright (c) 1994-2006 Sun Microsystems Inc.
 // All Rights Reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 // - Redistributions of source code must retain the above copyright notice,
 // this list of conditions and the following disclaimer.
 //
@@ skipping 834 matching lines @@
 
 // Helper class for generating code or data associated with the code
 // right after a call instruction. As an example this can be used to
 // generate safepoint data after calls for crankshaft.
 class CallWrapper {
  public:
   CallWrapper() { }
   virtual ~CallWrapper() { }
   // Called just before emitting a call. Argument is the size of the generated
   // call code.
-  virtual void BeforeCall(int call_size) = 0;
+  virtual void BeforeCall(int call_size) const = 0;
   // Called just after emitting a call, i.e., at the return site for the call.
-  virtual void AfterCall() = 0;
+  virtual void AfterCall() const = 0;
+};
+
+class NullCallWrapper : public CallWrapper {
+ public:
+  NullCallWrapper() { }
+  virtual ~NullCallWrapper() { }
+  virtual void BeforeCall(int call_size) const { }
+  virtual void AfterCall() const { }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_ASSEMBLER_H_