Empty @viewport should not override legacy viewport.

Source/core/css/resolver/ViewportStyleResolver.h

Index: Source/core/css/resolver/ViewportStyleResolver.h
diff --git a/Source/core/css/resolver/ViewportStyleResolver.h b/Source/core/css/resolver/ViewportStyleResolver.h
index b71128e9f0bc02d9f214a22d7c21f4ebb00646de..bb0a36ff9bb9ef27e4e4a1f67edeb911bbd7ce2f 100644
--- a/Source/core/css/resolver/ViewportStyleResolver.h
+++ b/Source/core/css/resolver/ViewportStyleResolver.h
@@ -31,6 +31,7 @@
 #define ViewportStyleResolver_h
 
 #include "CSSPropertyNames.h"
+#include "core/css/RuleSet.h"
 #include "core/platform/Length.h"
 #include "wtf/RefCounted.h"
 #include "wtf/RefPtr.h"
@@ -50,8 +51,9 @@ public:
 
     ~ViewportStyleResolver();
 
-    void setHasAuthorStyle() { m_hasAuthorStyle = true; }
-    void addViewportRule(StyleRuleViewport*);
+    enum Origin { UserAgentOrigin, AuthorOrigin };
+
+    void collectViewportRules(RuleSet*, Origin);
 
     void clearDocument();
     void resolve();
@@ -59,6 +61,8 @@ public:
 private:
     explicit ViewportStyleResolver(Document*);
 
+    void addViewportRule(StyleRuleViewport*, Origin);
+
     float viewportArgumentValue(CSSPropertyID) const;
     Length viewportLengthValue(CSSPropertyID) const;