Remove out-of-date FIXME on collapsing margins

Source/core/rendering/RenderBlockFlow.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 1280 matching lines @@
         // i.e., clearance = [height of float] - margin-top".
         setLogicalHeight(child->logicalTop() + childMargins.negativeMarginBefore());
     } else {
         // Increase our height by the amount we had to clear.
         setLogicalHeight(logicalHeight() + heightIncrease);
     }
 
     if (marginInfo.canCollapseWithMarginBefore()) {
         // We can no longer collapse with the top of the block since a clear
         // occurred. The empty blocks collapse into the cleared block.
-        // FIXME: This isn't quite correct. Need clarification for what to do
-        // if the height the cleared block is offset by is smaller than the
-        // margins involved.

[Julien - ping for review, 2014/03/19 00:56:24]

Removing this FIXME is OK but are we matching CSS 2.1's behavior? Is this
covered by any cases?

By removing this FIXME, we may be losing the fact that this code needs to be
updated. I am fine with either fixing (along with appropriate testing) or just
adding an up-to-date FIXME.

         setMaxMarginBeforeValues(oldTopPosMargin, oldTopNegMargin);
         marginInfo.setAtBeforeSideOfBlock(false);
 
         // In case the child discarded the before margin of the block we need to reset the mustDiscardMarginBefore flag to the initial value.
         setMustDiscardMarginBefore(style()->marginBeforeCollapse() == MDISCARD);
     }
 
     return yPos + heightIncrease;
 }
 
@@ skipping 1523 matching lines @@
 RenderBlockFlow::RenderBlockFlowRareData& RenderBlockFlow::ensureRareData()
 {
     if (m_rareData)
         return *m_rareData;
 
     m_rareData = adoptPtr(new RenderBlockFlowRareData(this));
     return *m_rareData;
 }
 
 } // namespace WebCore