[New Multicolumn] Make fast/multicol/overflow-content.html pass in the new implementation.

Source/core/rendering/RenderMultiColumnSet.cpp

 /*
  * Copyright (C) 2012 Apple 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 100 matching lines @@
             largestHeight = height;
             indexWithLargestHeight = i;
         }
         previousOffset = run.breakOffset();
     }
     return indexWithLargestHeight;
 }
 
 void RenderMultiColumnSet::distributeImplicitBreaks()
 {
-    unsigned breakCount = forcedBreaksCount();
-
 #ifndef NDEBUG
     // There should be no implicit breaks assumed at this point.
-    for (unsigned i = 0; i < breakCount; i++)
+    for (unsigned i = 0; i < forcedBreaksCount(); i++)
         ASSERT(!m_contentRuns[i].assumedImplicitBreaks());
 #endif // NDEBUG
 
-    // There will always be at least one break, since the flow thread reports a "forced break" at
-    // end of content.
-    ASSERT(breakCount >= 1);
+    // Insert a final content run to encompass all content. This will include overflow if this is
+    // the last set.
+    addForcedBreak(logicalBottomInFlowThread());

[Julien - ping for review, 2014/05/07 00:55:17]

Doesn't this make the call to applyBreakAfterContent in
RenderBlockFlow::layoutBlockFlow useless as we have already created a forced
break at the end of our content?

More generally, do we need applyBreakAfterContent or shouldn't we just do call
recalculateColumnHeight to make sure our content gets rebalanced appropriately?

[Julien - ping for review, 2014/05/07 01:01:11]

Forget this comment, I missed that this change had more than one file when
reviewing :-)

+    unsigned breakCount = forcedBreaksCount();
 
     // If there is room for more breaks (to reach the used value of column-count), imagine that we
     // insert implicit breaks at suitable locations. At any given time, the content run with the
     // currently tallest columns will get another implicit break "inserted", which will increase its
     // column count by one and shrink its columns' height. Repeat until we have the desired total
     // number of breaks. The largest column height among the runs will then be the initial column
     // height for the balancer to use.
     while (breakCount < m_computedColumnCount) {
         unsigned index = findRunWithTallestColumns();
         m_contentRuns[index].assumeAnotherImplicitBreak();
@@ skipping 462 matching lines @@
         fragments.append(fragment);
     }
 }
 
 const char* RenderMultiColumnSet::renderName() const
 {
     return "RenderMultiColumnSet";
 }
 
 }