Enable zero length source size in sizes attribute

Source/core/css/parser/SizesAttributeParser.h

Index: Source/core/css/parser/SizesAttributeParser.h
diff --git a/Source/core/css/parser/SizesAttributeParser.h b/Source/core/css/parser/SizesAttributeParser.h
index 098753d631258085551d8a0461fdc7f0168a244d..3c7bc4ee260c25283576e9169a1f6818161fe982 100644
--- a/Source/core/css/parser/SizesAttributeParser.h
+++ b/Source/core/css/parser/SizesAttributeParser.h
@@ -21,6 +21,7 @@ private:
     SizesAttributeParser(PassRefPtr<MediaValues> mediaValues)
         : m_mediaValues(mediaValues)
         , m_length(0)
+        , m_lengthWasSet(false)

[eseidel, 2014/06/02 16:24:38]

Entertained that Chris Dumez was just making one of these out-of-band bools
in-band in a separate CL.

     {
     }
 
@@ -34,6 +35,7 @@ private:
     RefPtrWillBeMember<MediaQuerySet> m_mediaCondition;
     RefPtr<MediaValues> m_mediaValues;
     unsigned m_length;
+    bool m_lengthWasSet;
 };
 
 } // namespace