Clean up forward declarations in Source/platform

Source/platform/Cookie.h

 /*
  * Copyright (C) 2009 Joseph Pecoraro. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 57 matching lines @@
     }
 
     static bool equal(Cookie a, Cookie b)
     {
         return a.name == b.name && a.domain == b.domain && a.path == b.path && a.secure == b.secure;
     }
 };
 } // namespace blink
 
 namespace WTF {
-template<typename T> struct DefaultHash;
 template<> struct DefaultHash<blink::Cookie> {
     typedef blink::CookieHash Hash;
 };
 }
 
 #endif