Add a FOUC painting test.

Source/core/frame/FrameView.h

 /*
    Copyright (C) 1997 Martin Jones (mjones@kde.org)
              (C) 1998 Waldo Bastian (bastian@kde.org)
              (C) 1998, 1999 Torben Weis (weis@kde.org)
              (C) 1999 Lars Knoll (knoll@kde.org)
              (C) 1999 Antti Koivisto (koivisto@kde.org)
    Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
 
    This library is free software; you can redistribute it and/or
    modify it under the terms of the GNU Library General Public
@@ skipping 270 matching lines @@
     void scrollbarVisibilityChanged() override;
 
     // FIXME: Remove this method once plugin loading is decoupled from layout.
     void flushAnyPendingPostLayoutTasks();
 
     bool shouldSuspendScrollAnimations() const override;
     void scrollbarStyleChanged() override;
 
     LayoutBox* embeddedContentBox() const;
 
+    static void setInitialTracksPaintInvalidationsForTesting(bool);
+
     void setTracksPaintInvalidations(bool);
     bool isTrackingPaintInvalidations() const { return m_isTrackingPaintInvalidations; }
     void resetTrackedPaintInvalidations();
 
     using ScrollableAreaSet = WillBeHeapHashSet<RawPtrWillBeMember<ScrollableArea>>;
     void addScrollableArea(ScrollableArea*);
     void removeScrollableArea(ScrollableArea*);
     const ScrollableAreaSet* scrollableAreas() const { return m_scrollableAreas.get(); }
 
     void addAnimatingScrollableArea(ScrollableArea*);
@@ skipping 557 matching lines @@
     static const unsigned visualPixelThreshold = 32 * 32;
     if (m_visuallyNonEmptyPixelCount > visualPixelThreshold)
         setIsVisuallyNonEmpty();
 }
 
 DEFINE_TYPE_CASTS(FrameView, Widget, widget, widget->isFrameView(), widget.isFrameView());
 
 } // namespace blink
 
 #endif // FrameView_h