Implement collection of @viewport rules from DocumentStyleCollection.

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 7db8515853dbe9bc47fbe4cf3499e819f3ce4dfc..5219819b4b6375a81e31bfaf3e40ae271aa731e1 100644
--- a/third_party/WebKit/Source/core/dom/DocumentStyleSheetCollection.h
+++ b/third_party/WebKit/Source/core/dom/DocumentStyleSheetCollection.h
@@ -38,6 +38,7 @@ namespace blink {
 class DocumentStyleSheetCollector;
 class StyleEngine;
 class TreeScope;
+class ViewportStyleResolver;
 
 class DocumentStyleSheetCollection final
     : public TreeScopeStyleSheetCollection {
@@ -50,6 +51,7 @@ class DocumentStyleSheetCollection final
 
   void updateActiveStyleSheets(StyleEngine&, StyleResolverUpdateMode);
   void collectStyleSheets(StyleEngine&, DocumentStyleSheetCollector&);
+  void collectViewportRules(ViewportStyleResolver&);
 
   DEFINE_INLINE_VIRTUAL_TRACE() {
     TreeScopeStyleSheetCollection::trace(visitor);