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 330a68586c537a53f3c131836951154241ede8b5..1c6b21c0c1473ffbaffcee7c52b950b6d4139642 100644
--- a/Source/core/page/Page.h
+++ b/Source/core/page/Page.h
@@ -254,6 +254,15 @@ public:
 
     double timerAlignmentInterval() const;
 
+    class MultisamplingChangedObserver {
+    public:
+        virtual void multisamplingChanged(bool) = 0;
+    };
+
+    void addMultisamplingChangedObserver(MultisamplingChangedObserver*);
+    void removeMultisamplingChangedObserver(MultisamplingChangedObserver*);
+    void multisamplingChanged();
+
 private:
     void initGroup();
 
@@ -333,6 +342,8 @@ private:
 #endif
 
     OwnPtr<PageConsole> m_console;
+
+    HashSet<MultisamplingChangedObserver*> m_multisamplingChangedObservers;
 };
 
 } // namespace WebCore