Avoid stretching the parent of a column spanner.

Source/core/layout/LayoutBlockFlow.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 995 matching lines @@
         }
         if (child->isColumnSpanAll()) {
             // This is not the containing block of the spanner. The spanner's placeholder will lay
             // it out in due course. For now we just need to consult our flow thread, so that the
             // columns (if any) preceding and following the spanner are laid out correctly. But
             // first we apply the pending margin, so that it's taken into consideration and doesn't
             // end up on the other side of the spanner.
             setLogicalHeight(logicalHeight() + marginInfo.margin());
             marginInfo.clearMargin();
 
-            LayoutUnit adjustment = flowThreadContainingBlock()->skipColumnSpanner(child, offsetFromLogicalTopOfFirstPage() + logicalHeight());
-            setLogicalHeight(logicalHeight() + adjustment);
+            flowThreadContainingBlock()->skipColumnSpanner(child, offsetFromLogicalTopOfFirstPage() + logicalHeight());
             continue;
         }
 
         // Lay out the child.
         layoutBlockChild(*child, marginInfo, previousFloatLogicalBottom);
         lastNormalFlowChild = child;
     }
 
     // Now do the handling of the bottom of the block, adding in our bottom border/padding and
     // determining the correct collapsed bottom margin information.
@@ skipping 2052 matching lines @@
     FrameView* frameView = document().view();
     LayoutUnit top = (style()->position() == FixedPosition) ? 0 : frameView->scrollOffset().height();
     int visibleHeight = frameView->visibleContentRect(IncludeScrollbars).height();
     if (size().height() < visibleHeight)
         top += (visibleHeight - size().height()) / 2;
     setY(top);
     dialog->setCentered(top);
 }
 
 } // namespace blink