[New Multicolumn] Handle flow thread overflow correctly in flipped writing modes.

Source/core/layout/MultiColumnFragmentainerGroup.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 
 #include "core/layout/MultiColumnFragmentainerGroup.h"
 
 #include "core/layout/LayoutMultiColumnSet.h"
 
@@ skipping 139 matching lines @@
 {
     // |layerBoundingBox| is in the flow thread coordinate space, relative to the top/left edge of
     // the flow thread, but note that it has been converted with respect to writing mode (so that
     // it's visual/physical in that sense).
     //
     // |dirtyRect| is visual, relative to the multicol container.
     //
     // Then there's the output from this method - the stuff we put into the list of fragments. The
     // fragment.paginationOffset point is the actual visual translation required to get from a
     // location in the flow thread to a location in a given column. The fragment.paginationClip
-    // rectangle, on the other hand, is in flow thread coordinates.
+    // rectangle, on the other hand, is in flow thread coordinates, but otherwise completely
+    // physical in terms of writing mode.
     //
     // All other rectangles in this method are sized physically, and the inline direction coordinate
     // is physical too, but the block direction coordinate is "logical top". This is the same as
     // e.g. RenderBox::frameRect(). These rectangles also pretend that there's only one long column,
     // i.e. they are for the flow thread.
 
     LayoutMultiColumnFlowThread* flowThread = m_columnSet.multiColumnFlowThread();
     bool isHorizontalWritingMode = m_columnSet.isHorizontalWritingMode();
 
     // Put the layer bounds into flow thread-local coordinates by flipping it first. Since we're in
@@ skipping 329 matching lines @@
     append(MultiColumnFragmentainerGroup(m_columnSet));
     return last();
 }
 
 void MultiColumnFragmentainerGroupList::deleteExtraGroups()
 {
     shrink(1);
 }
 
 } // namespace blink