Separate @viewport from other RuleSet construction.

third_party/WebKit/Source/core/dom/DocumentStyleSheetCollection.h

Index: third_party/WebKit/Source/core/dom/DocumentStyleSheetCollection.h
diff --git a/third_party/WebKit/Source/core/dom/DocumentStyleSheetCollection.h b/third_party/WebKit/Source/core/dom/DocumentStyleSheetCollection.h
index dd7c15a202f6daddb3a0fd259fa21007a33eaab2..f062aeb033556241c59737e9ecae5df85ea141b7 100644
--- a/third_party/WebKit/Source/core/dom/DocumentStyleSheetCollection.h
+++ b/third_party/WebKit/Source/core/dom/DocumentStyleSheetCollection.h
@@ -39,7 +39,7 @@ class DocumentStyleSheetCollector;
 class StyleEngine;
 class TreeScope;
 
-class DocumentStyleSheetCollection final
+class CORE_EXPORT DocumentStyleSheetCollection final
     : public TreeScopeStyleSheetCollection {
   WTF_MAKE_NONCOPYABLE(DocumentStyleSheetCollection);
 
@@ -50,6 +50,7 @@ class DocumentStyleSheetCollection final
 
   void updateActiveStyleSheets(StyleEngine&, StyleResolverUpdateMode);
   void collectStyleSheets(StyleEngine&, DocumentStyleSheetCollector&);
+  void updateViewport();
 
   DEFINE_INLINE_VIRTUAL_TRACE() {
     TreeScopeStyleSheetCollection::trace(visitor);