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

Source/core/layout/BidiRun.h

Index: Source/core/layout/BidiRun.h
diff --git a/Source/core/layout/BidiRun.h b/Source/core/layout/BidiRun.h
index 44bda444f6d5be439ff958aa62c36e9dbeda61a0..a325bf5b48d5ad35f56336f0cc1a39b296a88ddf 100644
--- a/Source/core/layout/BidiRun.h
+++ b/Source/core/layout/BidiRun.h
@@ -37,7 +37,7 @@ struct BidiRun : BidiCharacterRun {
     BidiRun(int start, int stop, LayoutObject* object, BidiContext* context, WTF::Unicode::Direction dir)
         : BidiCharacterRun(start, stop, context, dir)
         , m_object(object)
-        , m_box(0)
+        , m_box(nullptr)
     {
         // Stored in base class to save space.
         m_hasHyphen = false;