Add Frame::isMainFrame and deploy it

Source/core/rendering/CompositedLayerMapping.cpp

Index: Source/core/rendering/CompositedLayerMapping.cpp
diff --git a/Source/core/rendering/CompositedLayerMapping.cpp b/Source/core/rendering/CompositedLayerMapping.cpp
index 8a061b5a8dcc0642bbc6967acc61b5ae58051c86..92c923734c556bd8ba93e3ac8b9ceaade988306e 100644
--- a/Source/core/rendering/CompositedLayerMapping.cpp
+++ b/Source/core/rendering/CompositedLayerMapping.cpp
@@ -167,13 +167,8 @@ CompositedLayerMapping::CompositedLayerMapping(RenderLayer* layer)
     , m_canCompositeFilters(false)
     , m_backgroundLayerPaintsFixedRootBackground(false)
 {
-    if (layer->isRootLayer()) {
-        Frame& frame = toRenderView(renderer())->frameView()->frame();
-        Page* page = frame.page();
-        if (page && page->mainFrame() == &frame) {
-            m_isMainFrameRenderViewLayer = true;
-        }
-    }
+    if (layer->isRootLayer() && renderer()->frame()->isMainFrame())
+        m_isMainFrameRenderViewLayer = true;
 
     createPrimaryGraphicsLayer();
 }