Clean up forward declarations in Source/platform

Source/platform/weborigin/SecurityOriginHash.h

 /*
  * Copyright (C) 2008 Apple 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:
  *
  * 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
@@ skipping 71 matching lines @@
         return equal(a.get(), b.get());
     }
 
     static const bool safeToCompareToEmptyOrDeleted = false;
 };
 
 } // namespace blink
 
 namespace WTF {
 
-template<typename> struct DefaultHash;
-
 template<> struct DefaultHash<RefPtr<blink::SecurityOrigin> > {
     typedef blink::SecurityOriginHash Hash;
 };
 
 } // namespace WTF
 
 #endif // SecurityOriginHash_h