[New Multicolumn] Update sets' flow thread portion rectangles properly.

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 01711123b4832c8d39ff2a6cd0ea71dad9bf1853..4d35a651c87e5561d1deca824157fc034e2094c2 100644
--- a/Source/core/rendering/RenderMultiColumnSet.cpp
+++ b/Source/core/rendering/RenderMultiColumnSet.cpp
@@ -286,6 +286,16 @@ void RenderMultiColumnSet::prepareForLayout()
     m_minimumColumnHeight = 0;
 }
 
+void RenderMultiColumnSet::layout()
+{
+    RenderRegion::layout();
+
+    if (!nextSiblingMultiColumnSet()) {
+        // This is the last set, i.e. the last region. Seize the opportunity to validate them.
+        multiColumnFlowThread()->validateRegions();

[Julien - ping for review, 2014/05/06 23:46:00]

RenderFlowThread::layout calls this function already so why would we need to
call it twice?

[mstensho (USE GERRIT), 2014/05/07 07:35:11]

Done.

+    }
+}
+
 void RenderMultiColumnSet::computeLogicalHeight(LayoutUnit, LayoutUnit logicalTop, LogicalExtentComputedValues& computedValues) const
 {
     computedValues.m_extent = m_computedColumnHeight;