Empty @viewport should not override legacy viewport.

Source/core/css/resolver/ViewportStyleResolver.h

 /*
  * Copyright (C) 2012 Intel Corporation. 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.
@@ skipping 13 matching lines @@
  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR
  * TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
  * THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
  */
 
 #ifndef ViewportStyleResolver_h
 #define ViewportStyleResolver_h
 
 #include "CSSPropertyNames.h"
+#include "core/css/RuleSet.h"
 #include "core/platform/Length.h"
 #include "wtf/RefCounted.h"
 #include "wtf/RefPtr.h"
 
 namespace WebCore {
 
 class Document;
 class MutableStylePropertySet;
 class StyleRuleViewport;
 
 class ViewportStyleResolver : public RefCounted<ViewportStyleResolver> {
 public:
     static PassRefPtr<ViewportStyleResolver> create(Document* document)
     {
         return adoptRef(new ViewportStyleResolver(document));
     }
 
     ~ViewportStyleResolver();
 
-    void setHasAuthorStyle() { m_hasAuthorStyle = true; }
-    void addViewportRule(StyleRuleViewport*);
+    enum Origin { UserAgentOrigin, AuthorOrigin };
+
+    void collectViewportRules(RuleSet*, Origin);
 
     void clearDocument();
     void resolve();
 
 private:
     explicit ViewportStyleResolver(Document*);
 
+    void addViewportRule(StyleRuleViewport*, Origin);
+
     float viewportArgumentValue(CSSPropertyID) const;
     Length viewportLengthValue(CSSPropertyID) const;
 
     Document* m_document;
     RefPtr<MutableStylePropertySet> m_propertySet;
     bool m_hasAuthorStyle;
 };
 
 } // namespace WebCore
 
 #endif // ViewportStyleResolver_h