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

Source/core/layout/BidiRunForLine.h

 /*
  * Copyright (C) 2000 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2003, 2004, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc.
  * All right reserved.
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 12 matching lines @@
 
 #ifndef BidiRunForLine_h
 #define BidiRunForLine_h
 
 #include "core/layout/line/TrailingObjects.h"
 #include "platform/text/BidiResolver.h"
 
 namespace blink {
 
 TextDirection determinePlaintextDirectionality(LayoutObject* root,
-    LayoutObject* current = 0, unsigned pos = 0);
+    LayoutObject* current = nullptr, unsigned pos = 0);
 
 void constructBidiRunsForLine(InlineBidiResolver&, BidiRunList<BidiRun>&,
     const InlineIterator& endOfLine, VisualDirectionOverride,
     bool previousLineBrokeCleanly, bool isNewUBAParagraph);
 
 } // namespace blink
 
 #endif // BidiRunForLine_h