Change Page::m_mainFrame to be a Frame.

Source/core/frame/PinchViewport.cpp

Index: Source/core/frame/PinchViewport.cpp
diff --git a/Source/core/frame/PinchViewport.cpp b/Source/core/frame/PinchViewport.cpp
index c11f62758b939c50674fe58fb3d17ca72bfc6a68..ce5ea3a25ce209d3123ee7d79d2c337c5aa1f548 100644
--- a/Source/core/frame/PinchViewport.cpp
+++ b/Source/core/frame/PinchViewport.cpp
@@ -313,9 +313,10 @@ void PinchViewport::registerLayersWithTreeView(WebLayerTreeView* layerTreeView)
     TRACE_EVENT0("webkit", "PinchViewport::registerLayersWithTreeView");
     ASSERT(layerTreeView);
     ASSERT(m_frameHost.page().mainFrame());
-    ASSERT(m_frameHost.page().mainFrame()->contentRenderer());
+    ASSERT(m_frameHost.page().mainFrame()->isLocalFrame());
+    ASSERT(toLocalFrame(m_frameHost.page().mainFrame())->contentRenderer());
 
-    RenderLayerCompositor* compositor = m_frameHost.page().mainFrame()->contentRenderer()->compositor();
+    RenderLayerCompositor* compositor = toLocalFrame(m_frameHost.page().mainFrame())->contentRenderer()->compositor();
     // Get the outer viewport scroll layer.
     WebLayer* scrollLayer = compositor->scrollLayer()->platformLayer();
 
@@ -418,7 +419,7 @@ void PinchViewport::paintContents(const GraphicsLayer*, GraphicsContext&, Graphi
 
 LocalFrame* PinchViewport::mainFrame() const
 {
-    return m_frameHost.page().mainFrame();
+    return m_frameHost.page().mainFrame() && m_frameHost.page().mainFrame()->isLocalFrame() ? toLocalFrame(m_frameHost.page().mainFrame()) : 0;
 }
 
 FloatPoint PinchViewport::clampOffsetToBoundaries(const FloatPoint& offset)