Add Frame::isMainFrame and deploy it

Source/bindings/v8/V8WindowShell.cpp

Index: Source/bindings/v8/V8WindowShell.cpp
diff --git a/Source/bindings/v8/V8WindowShell.cpp b/Source/bindings/v8/V8WindowShell.cpp
index 7d9ca86d0a5f7635976b5f3e2b7efd5f3d7286cd..c114d2dbdca09b8b083478802f5a8a7a75321b08 100644
--- a/Source/bindings/v8/V8WindowShell.cpp
+++ b/Source/bindings/v8/V8WindowShell.cpp
@@ -105,8 +105,7 @@ void V8WindowShell::disposeContext()
     // It's likely that disposing the context has created a lot of
     // garbage. Notify V8 about this so it'll have a chance of cleaning
     // it up when idle.
-    bool isMainFrame = m_frame->page() && (m_frame->page()->mainFrame() == m_frame);
-    V8GCForContextDispose::instance().notifyContextDisposed(isMainFrame);
+    V8GCForContextDispose::instance().notifyContextDisposed(m_frame->isMainFrame());
 }
 
 void V8WindowShell::clearForClose(bool destroyGlobal)