Fix scroll origin, overflow rects, and coordinate flipping for flexbox.

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@gmail.com>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 574 matching lines @@
 int PaintLayerScrollableArea::pixelSnappedScrollHeight() const
 {
     return snapSizeToPixel(scrollHeight(), box().clientTop() + box().location().y());
 }
 
 void PaintLayerScrollableArea::computeScrollDimensions()
 {
     m_overflowRect = box().layoutOverflowRect();
     box().flipForWritingMode(m_overflowRect);
 
-    int scrollableLeftOverflow = m_overflowRect.x() - box().borderLeft() - (box().shouldPlaceBlockDirectionScrollbarOnLogicalLeft() ? box().verticalScrollbarWidth() : 0);
-    int scrollableTopOverflow = m_overflowRect.y() - box().borderTop();
-    setScrollOrigin(IntPoint(-scrollableLeftOverflow, -scrollableTopOverflow));
+    LayoutPoint scrollableOverflow = m_overflowRect.location() - LayoutSize(box().borderLeft(), box().borderTop());
+    setScrollOrigin(flooredIntPoint(-scrollableOverflow) + box().originAdjustmentForScrollbars());
 }
 
 void PaintLayerScrollableArea::scrollToPosition(const DoublePoint& scrollPosition, ScrollOffsetClamping clamp, ScrollBehavior scrollBehavior, ScrollType scrollType)
 {
     cancelProgrammaticScrollAnimation();
 
     DoublePoint newScrollPosition = clamp == ScrollOffsetClamped ? clampScrollPosition(scrollPosition) : scrollPosition;
     if (newScrollPosition != scrollPositionDouble())
         ScrollableArea::setScrollPosition(newScrollPosition, scrollType, scrollBehavior);
 }
@@ skipping 1001 matching lines @@
 }
 
 DEFINE_TRACE(PaintLayerScrollableArea::ScrollbarManager)
 {
     visitor->trace(m_scrollableArea);
     visitor->trace(m_hBar);
     visitor->trace(m_vBar);
 }
 
 } // namespace blink