Changes to ALLOW_UNUSED to match upcoming changes to the Chromium trunk:

src/base/compiler-specific.h

 // Copyright 2014 the V8 project 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 V8_BASE_COMPILER_SPECIFIC_H_
 #define V8_BASE_COMPILER_SPECIFIC_H_
 
 #include "include/v8config.h"
 
-// Annotate a variable indicating it's ok if the variable is not used.
-// (Typically used to silence a compiler warning when the assignment
-// is important for some other reason.)
+// Annotate a typedef or function indicating it's ok if it's not used.
 // Use like:
-//   int x ALLOW_UNUSED = ...;
+//   typedef Foo Bar ALLOW_UNUSED_TYPE;
 #if V8_HAS_ATTRIBUTE_UNUSED
-#define ALLOW_UNUSED __attribute__((unused))
+#define ALLOW_UNUSED_TYPE __attribute__((unused))
 #else
-#define ALLOW_UNUSED
+#define ALLOW_UNUSED_TYPE
 #endif
 
 
 // Annotate a virtual method indicating it must be overriding a virtual
 // method in the parent class.
 // Use like:
 //   virtual void bar() OVERRIDE;
 #if V8_HAS_CXX11_OVERRIDE
 #define OVERRIDE override
 #else
@@ skipping 20 matching lines @@
 // Annotate a function indicating the caller must examine the return value.
 // Use like:
 //   int foo() WARN_UNUSED_RESULT;
 #if V8_HAS_ATTRIBUTE_WARN_UNUSED_RESULT
 #define WARN_UNUSED_RESULT __attribute__((warn_unused_result))
 #else
 #define WARN_UNUSED_RESULT /* NOT SUPPORTED */
 #endif
 
 #endif  // V8_BASE_COMPILER_SPECIFIC_H_