Enable zero length source size in sizes attribute

Source/core/css/parser/SizesAttributeParser.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SizesAttributeParser_h
 #define SizesAttributeParser_h
 
 #include "core/css/MediaValues.h"
 #include "core/css/parser/MediaQueryParser.h"
 #include "platform/heap/Handle.h"
 #include "wtf/text/WTFString.h"
 
 namespace WebCore {
 
 class SizesAttributeParser {
     STACK_ALLOCATED();
 public:
     static unsigned findEffectiveSize(const String& attribute, PassRefPtr<MediaValues>);
 
 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.

     {
     }
 
     bool parse(Vector<MediaQueryToken>& tokens);
     bool parseMediaConditionAndLength(MediaQueryTokenIterator startToken, MediaQueryTokenIterator endToken);
     unsigned effectiveSize();
     bool calculateLengthInPixels(MediaQueryTokenIterator startToken, MediaQueryTokenIterator endToken, unsigned& result);
     bool mediaConditionMatches(PassRefPtrWillBeRawPtr<MediaQuerySet> mediaCondition);
     unsigned effectiveSizeDefaultValue();
 
     RefPtrWillBeMember<MediaQuerySet> m_mediaCondition;
     RefPtr<MediaValues> m_mediaValues;
     unsigned m_length;
+    bool m_lengthWasSet;
 };
 
 } // namespace
 
 #endif