Change Page::m_mainFrame to be a Frame.

Source/web/tests/MHTMLTest.cpp

 /*
  * Copyright (C) 2014 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 84 matching lines @@
 // generated from.
 TEST_F(MHTMLTest, CheckDomain)
 {
     const char kFileURL[] = "file:///simple_test.mht";
 
     // Register the mocked frame and load it.
     WebURL url = toKURL(kFileURL);
     registerMockedURLLoad(kFileURL, WebString::fromUTF8("simple_test.mht"));
     loadURLInTopFrame(url);
     ASSERT_TRUE(page());
-    LocalFrame* frame = page()->mainFrame();
+    LocalFrame* frame = toLocalFrame(page()->mainFrame());
     ASSERT_TRUE(frame);
     Document* document = frame->document();
     ASSERT_TRUE(document);
 
     EXPECT_STREQ(kFileURL, frame->domWindow()->location().href().ascii().data());
 
     WebCore::SecurityOrigin* origin = document->securityOrigin();
     EXPECT_STRNE("localhost", origin->domain().ascii().data());
 }
 
 }