Clean up forward declarations in Source/platform

Source/platform/fonts/WidthCache.h

 /*
  * Copyright (C) 2012 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 18 matching lines @@
 #include "platform/geometry/IntRectExtent.h"
 #include "platform/text/TextRun.h"
 #include "wtf/Forward.h"
 #include "wtf/HashFunctions.h"
 #include "wtf/HashSet.h"
 #include "wtf/HashTableDeletedValueType.h"
 #include "wtf/StringHasher.h"
 
 namespace blink {
 
-struct GlyphOverflow;
-
 struct WidthCacheEntry {
     WidthCacheEntry()
     {
         width = std::numeric_limits<float>::quiet_NaN();
     }
     bool isValid() const { return !std::isnan(width); }
     float width;
     IntRectExtent glyphBounds;
 };
 
@@ skipping 155 matching lines @@
 inline bool operator==(const WidthCache::SmallStringKey& a, const WidthCache::SmallStringKey& b)
 {
     if (a.length() != b.length())
         return false;
     return WTF::equal(a.characters(), b.characters(), a.length());
 }
 
 } // namespace blink
 
 #endif // WidthCache_h