Change Page::m_mainFrame to be a Frame.

Source/core/rendering/FastTextAutosizer.cpp

Index: Source/core/rendering/FastTextAutosizer.cpp
diff --git a/Source/core/rendering/FastTextAutosizer.cpp b/Source/core/rendering/FastTextAutosizer.cpp
index 0e22dc763473827f82cd1aef7c11f0f658cc1dde..d41ee2da60ab833c3a3311e278d449a31e5c5b55 100644
--- a/Source/core/rendering/FastTextAutosizer.cpp
+++ b/Source/core/rendering/FastTextAutosizer.cpp
@@ -513,7 +513,7 @@ void FastTextAutosizer::updatePageInfo()
         RenderView* renderView = m_document->renderView();
         bool horizontalWritingMode = isHorizontalWritingMode(renderView->style()->writingMode());
 
-        LocalFrame* mainFrame = m_document->page()->mainFrame();
+        LocalFrame* mainFrame = toLocalFrame(m_document->page()->mainFrame());
         IntSize frameSize = m_document->settings()->textAutosizingWindowSizeOverride();
         if (frameSize.isEmpty())
             frameSize = mainFrame->view()->unscaledVisibleContentSize(IncludeScrollbars);
@@ -1116,7 +1116,7 @@ FastTextAutosizer::TableLayoutScope::TableLayoutScope(RenderTable* table)
 }
 
 FastTextAutosizer::DeferUpdatePageInfo::DeferUpdatePageInfo(Page* page)
-    : m_mainFrame(page->mainFrame())
+    : m_mainFrame(toLocalFrame(page->mainFrame()))
 {
     if (FastTextAutosizer* textAutosizer = m_mainFrame->document()->fastTextAutosizer()) {
         ASSERT(!textAutosizer->m_updatePageInfoDeferred);