Add Frame::isMainFrame and deploy it

Source/core/rendering/RenderView.cpp

Index: Source/core/rendering/RenderView.cpp
diff --git a/Source/core/rendering/RenderView.cpp b/Source/core/rendering/RenderView.cpp
index cc036a720276d8a11ee3f4b7d03a4cb13b50c55d..f0f894e2f5e2926d488db69bba128d995f857972 100644
--- a/Source/core/rendering/RenderView.cpp
+++ b/Source/core/rendering/RenderView.cpp
@@ -1209,9 +1209,7 @@ IntervalArena* RenderView::intervalArena()
 bool RenderView::backgroundIsKnownToBeOpaqueInRect(const LayoutRect&) const
 {
     // FIXME: Remove this main frame check. Same concept applies to subframes too.
-    Page* page = document().page();
-    Frame* mainFrame = page ? page->mainFrame() : 0;
-    if (!m_frameView || &m_frameView->frame() != mainFrame)
+    if (!m_frameView || !m_frameView->isMainFrame())
         return false;
 
     return m_frameView->hasOpaqueBackground();