Get rid of clampScrollOffset and make maximumScrollPosition sane.

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 318 matching lines @@
         m_tickmarks = tickmarks;
         invalidatePaintForTickmarks();
     }
 
     void invalidatePaintForTickmarks() const;
 
     // Since the compositor can resize the viewport due to top controls and
     // commit scroll offsets before a WebView::resize occurs, we need to adjust
     // our scroll extents to prevent clamping the scroll offsets.
     void setTopControlsViewportAdjustment(float);
+    IntSize topControlsSize() const { return IntSize(0, roundf(m_topControlsViewportAdjustment)); }
 
     IntPoint maximumScrollPosition() const override;
 
     // ScrollableArea interface
     void invalidateScrollbarRect(Scrollbar*, const IntRect&) override;
     void getTickmarks(Vector<IntRect>&) const override;
     void scrollTo(const DoublePoint&);
     IntRect scrollableAreaBoundingBox() const override;
     bool scrollAnimatorEnabled() const override;
     bool usesCompositedScrolling() const override;
@@ skipping 508 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