Support OVERRIDE for clang.

base/compiler_specific.h

 // Copyright (c) 2006-2008 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 BASE_COMPILER_SPECIFIC_H_
 #define BASE_COMPILER_SPECIFIC_H_
 #pragma once
 
 #include "build/build_config.h"
 
@@ skipping 66 matching lines @@
 #else
 #define ALLOW_UNUSED
 #endif
 
 // Annotate a virtual method indicating it must be overriding a virtual
 // method in the parent class.
 // Use like:
 //   virtual void foo() OVERRIDE;
 #if defined(COMPILER_MSVC)
 #define OVERRIDE override
+#elif defined(__clang__)
+#define OVERRIDE __attribute__((override))
 #else
 #define OVERRIDE
 #endif
 
 // Annotate a function indicating the caller must examine the return value.
 // Use like:
 //   int foo() WARN_UNUSED_RESULT;
 #if defined(COMPILER_GCC)
 #define WARN_UNUSED_RESULT __attribute__((warn_unused_result))
 #else
@@ skipping 13 matching lines @@
 #endif
 
 // WPRINTF_FORMAT is the same, but for wide format strings.
 // This doesn't appear to yet be implemented in any compiler.
 // See http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38308 .
 #define WPRINTF_FORMAT(format_param, dots_param)
 // If available, it would look like:
 //   __attribute__((format(wprintf, format_param, dots_param)))
 
 #endif  // BASE_COMPILER_SPECIFIC_H_