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

Source/core/rendering/RenderMultiColumnSet.cpp

Index: Source/core/rendering/RenderMultiColumnSet.cpp
diff --git a/Source/core/rendering/RenderMultiColumnSet.cpp b/Source/core/rendering/RenderMultiColumnSet.cpp
index cfb9081a9850976c7a353f376a8252eca79b6ac5..24dffefc8c6fba4254dd511de54be91d2c8da66e 100644
--- a/Source/core/rendering/RenderMultiColumnSet.cpp
+++ b/Source/core/rendering/RenderMultiColumnSet.cpp
@@ -118,17 +118,16 @@ unsigned RenderMultiColumnSet::findRunWithTallestColumns() const
 
 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());
+    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