Convert the remaining MOJO_OVERRIDE -> override, and get rid of MOJO_OVERRIDE.

mojo/public/cpp/system/macros.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MOJO_PUBLIC_CPP_SYSTEM_MACROS_H_
 #define MOJO_PUBLIC_CPP_SYSTEM_MACROS_H_
 
 #include "mojo/public/c/system/macros.h"
 
 // Define a set of C++ specific macros.
 // Mojo C++ API users can assume that mojo/public/cpp/system/macros.h
 // includes mojo/public/c/system/macros.h.
 
-// TODO(vtl): Get rid of this once all users have been eliminated.
-// Annotate a virtual method indicating it must be overriding a virtual method
-// in the parent class. Use like:
-//   virtual void foo() OVERRIDE;
-#define MOJO_OVERRIDE override
-
 // A macro to disallow the copy constructor and operator= functions.
 // This should be used in the private: declarations for a class.
 #define MOJO_DISALLOW_COPY_AND_ASSIGN(TypeName) \
     TypeName(const TypeName&); \
     void operator=(const TypeName&)
 
 // Used to calculate the number of elements in an array.
 // (See |arraysize()| in Chromium's base/basictypes.h for more details.)
 namespace mojo {
 template <typename T, size_t N>
@@ skipping 15 matching lines @@
     }; \
     type(type&); \
     void operator=(type&); \
    public: \
     operator rvalue_type() { return rvalue_type(this); } \
     type Pass() { return type(rvalue_type(this)); } \
     typedef void MoveOnlyTypeForCPP03; \
    private:
 
 #endif  // MOJO_PUBLIC_CPP_SYSTEM_MACROS_H_