Have srcset respond to viewport changes

Source/core/dom/Document.h

Index: Source/core/dom/Document.h
diff --git a/Source/core/dom/Document.h b/Source/core/dom/Document.h
index 68fd2fa035b76ca0bdf4a49546a62975c8e83ced..45452d2e1d0471f96f37d65afab63d0d6604f0cb 100644
--- a/Source/core/dom/Document.h
+++ b/Source/core/dom/Document.h
@@ -149,6 +149,7 @@ class TouchList;
 class TransformSource;
 class TreeWalker;
 class VisitedLinkState;
+class ViewportChangeNotifier;
 class WebGLRenderingContext;
 
 struct AnnotatedRegionValue;
@@ -1033,6 +1034,8 @@ public:
     bool hasSVGFilterElementsRequiringLayerUpdate() const { return m_layerUpdateSVGFilterElements.size(); }
     void didRecalculateStyleForElement() { ++m_styleRecalcElementCounter; }
 
+    ViewportChangeNotifier& viewportChangeNotifier() { return *m_viewportChangeNotifier; }
+
 protected:
     Document(const DocumentInit&, DocumentClassFlags = DefaultDocumentClass);
 
@@ -1325,6 +1328,7 @@ private:
     bool m_writingModeSetOnDocumentElement;
     DocumentTiming m_documentTiming;
     RefPtrWillBeMember<MediaQueryMatcher> m_mediaQueryMatcher;
+    OwnPtrWillBeMember<ViewportChangeNotifier> m_viewportChangeNotifier;
     bool m_writeRecursionIsTooDeep;
     unsigned m_writeRecursionDepth;