Refactor srcset parser to align it with spec changes

Source/platform/ParsingUtilities.h

Index: Source/platform/ParsingUtilities.h
diff --git a/Source/platform/ParsingUtilities.h b/Source/platform/ParsingUtilities.h
index ba8fa9e55295f76346b56a02d88aa22d2f858a3b..882b800bf398e9e951aa39e1ff22bf40c48243ad 100644
--- a/Source/platform/ParsingUtilities.h
+++ b/Source/platform/ParsingUtilities.h
@@ -72,5 +72,12 @@ void skipWhile(const CharType*& position, const CharType* end)
         ++position;
 }
 
+template<typename CharType, bool characterPredicate(CharType)>
+void reverseSkipWhile(const CharType*& position, const CharType* start)
+{
+    while (position >= start && characterPredicate(*position))
+        --position;
+}
+
 #endif