Lose/restore WebGL contexts if multisampling blackist status changes at runtime.

Source/core/page/Page.h

Index: Source/core/page/Page.h
diff --git a/Source/core/page/Page.h b/Source/core/page/Page.h
index 879156f1a67410b2168bb81ad25f24d1aa188716..4aa957678f94695bd593726af55e58b3f80f8fc6 100644
--- a/Source/core/page/Page.h
+++ b/Source/core/page/Page.h
@@ -258,6 +258,15 @@ public:
 
     double timerAlignmentInterval() const;
 
+    class MultisamplingChangedObserver {
+    public:
+        virtual void multisamplingChanged(bool) = 0;
+    };
+
+    void addMultisamplingChangedObserver(MultisamplingChangedObserver*);
+    void removeMultisamplingChangedObserver(MultisamplingChangedObserver*);
+    void multisamplingChanged();
+
 private:
     void initGroup();
 
@@ -338,6 +347,8 @@ private:
     AlternativeTextClient* m_alternativeTextClient;
 
     OwnPtr<PageConsole> m_console;
+
+    HashSet<MultisamplingChangedObserver*> m_multisamplingChangedObservers;
 };
 
 } // namespace WebCore