Refactor srcset parser to align it with spec changes

Source/platform/ParsingUtilities.h

 /*
  * Copyright (C) 2013 Google 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 54 matching lines @@
         ++position;
 }
 
 template<typename CharType, bool characterPredicate(CharType)>
 void skipWhile(const CharType*& position, const CharType* end)
 {
     while (position < end && characterPredicate(*position))
         ++position;
 }
 
+template<typename CharType, bool characterPredicate(CharType)>
+void reverseSkipWhile(const CharType*& position, const CharType* start)
+{
+    while (position >= start && characterPredicate(*position))
+        --position;
+}
+
 #endif