C++11: Replace 0 with nullptr where applicable in layout code.

Source/core/layout/LayoutFlowThread.h

Index: Source/core/layout/LayoutFlowThread.h
diff --git a/Source/core/layout/LayoutFlowThread.h b/Source/core/layout/LayoutFlowThread.h
index 4377112b37ccaba95aa2f1e8aa34ce21afca4853..c7f59f947fe7504f5870a72805e7f3d7a63e3fb7 100644
--- a/Source/core/layout/LayoutFlowThread.h
+++ b/Source/core/layout/LayoutFlowThread.h
@@ -97,7 +97,7 @@ public:
 
     bool columnSetsHaveUniformLogicalHeight() const { return m_columnSetsHaveUniformLogicalHeight; }
 
-    virtual bool addForcedColumnBreak(LayoutUnit, LayoutObject* breakChild, bool isBefore, LayoutUnit* offsetBreakAdjustment = 0) { return false; }
+    virtual bool addForcedColumnBreak(LayoutUnit, LayoutObject* breakChild, bool isBefore, LayoutUnit* offsetBreakAdjustment = nullptr) { return false; }
 
     virtual bool isPageLogicalHeightKnown() const { return true; }
     bool pageLogicalSizeChanged() const { return m_pageLogicalSizeChanged; }
@@ -134,7 +134,7 @@ protected:
     public:
         MultiColumnSetSearchAdapter(LayoutUnit offset)
             : m_offset(offset)
-            , m_result(0)
+            , m_result(nullptr)
         {
         }