Need to notify the multicol machinery when floats have been laid out.

third_party/WebKit/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 3719 matching lines @@
 
   SubtreeLayoutScope layoutScope(child);
 
   // A new position may mean that we need to insert, move or remove breaks
   // inside the float. We may also need to lay out if we just ceased to be
   // fragmented, in order to remove pagination struts inside the child.
   markChildForPaginationRelayoutIfNeeded(child, layoutScope);
   child.layoutIfNeeded();
 
   if (isPaginated) {
+    paginatedContentWasLaidOut(child.logicalBottom());
+
     // We may have to insert a break before the float.
     LayoutUnit newLogicalTopMarginEdge =
         adjustFloatLogicalTopForPagination(child, logicalTopMarginEdge);
     if (logicalTopMarginEdge != newLogicalTopMarginEdge) {
       // We had already found a location for the float, but a soft
       // fragmentainer break then made us push it further down. This may affect
       // the inline position of the float (since we may no longer be beside the
       // same floats anymore). Block position will remain unaffected, though.
       floatLogicalLocation = computeLogicalLocationForFloat(
           floatingObject, newLogicalTopMarginEdge);
       DCHECK_EQ(floatLogicalLocation.y(), newLogicalTopMarginEdge);
       logicalTopMarginEdge = newLogicalTopMarginEdge;
 
       setLogicalTopForChild(child, logicalTopMarginEdge + marginBefore);
 
       // Pushing the child to the next fragmentainer most likely means that we
       // need to recalculate pagination struts inside it.
       if (child.isLayoutBlock())
         child.setChildNeedsLayout(MarkOnlyThis);
       child.layoutIfNeeded();
+      paginatedContentWasLaidOut(child.logicalBottom());
     }
   }
 
   LayoutUnit childLogicalLeftMargin =
       style()->isLeftToRightDirection() ? marginStart : marginEnd;
   setLogicalLeftForChild(child,
                          floatLogicalLocation.x() + childLogicalLeftMargin);
   setLogicalLeftForFloat(floatingObject, floatLogicalLocation.x());
   setLogicalTopForFloat(floatingObject, logicalTopMarginEdge);
   setLogicalHeightForFloat(floatingObject, logicalHeightForChild(child) +
@@ skipping 835 matching lines @@
   return LayoutBlock::invalidatePaintIfNeeded(paintInvalidationState);
 }
 
 void LayoutBlockFlow::invalidateDisplayItemClients(
     PaintInvalidationReason invalidationReason) const {
   BlockFlowPaintInvalidator(*this).invalidateDisplayItemClients(
       invalidationReason);
 }
 
 }  // namespace blink