Change Page::m_mainFrame to be a Frame.

Source/core/frame/FrameView.cpp

Index: Source/core/frame/FrameView.cpp
diff --git a/Source/core/frame/FrameView.cpp b/Source/core/frame/FrameView.cpp
index 884f4a828b16ad0db50479be94e6bc016dedda21..94eded8fd4b90cf7655a8043546ddf47f0ef6fc8 100644
--- a/Source/core/frame/FrameView.cpp
+++ b/Source/core/frame/FrameView.cpp
@@ -1199,7 +1199,7 @@ bool FrameView::useSlowRepaints(bool considerOverlap) const
 
     // The chromium compositor does not support scrolling a non-composited frame within a composited page through
     // the fast scrolling path, so force slow scrolling in that case.
-    if (m_frame->owner() && !hasCompositedContent() && m_frame->page() && m_frame->page()->mainFrame()->view()->hasCompositedContent())
+    if (m_frame->owner() && !hasCompositedContent() && m_frame->page() && m_frame->page()->mainFrame()->isLocalFrame() && toLocalFrame(m_frame->page()->mainFrame())->view()->hasCompositedContent())
         return true;
 
     if (m_isOverlapped && considerOverlap)