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 d58734140d43ce0c6dcfb8c065bed6c6cd680450..c4c6e14d8f2c5f391b2a38c65c1e05f595bd1fe0 100644
--- a/Source/core/page/Page.h
+++ b/Source/core/page/Page.h
@@ -259,6 +259,16 @@ public:
 
     double timerAlignmentInterval() const;
 
+    class MultisamplingChangedObserver {
+    public:
+        virtual void multisamplingChanged(bool) = 0;
+        virtual ~MultisamplingChangedObserver() {}
+    };
+
+    void addMultisamplingChangedObserver(MultisamplingChangedObserver*);
+    void removeMultisamplingChangedObserver(MultisamplingChangedObserver*);
+    void multisamplingChanged();
+
 private:
     void initGroup();
 
@@ -339,6 +349,8 @@ private:
     AlternativeTextClient* m_alternativeTextClient;
 
     OwnPtr<PageConsole> m_console;
+
+    HashSet<MultisamplingChangedObserver*> m_multisamplingChangedObservers;
 };
 
 } // namespace WebCore