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

Source/core/layout/LayoutTextControlMultiLine.cpp

 /**
  * Copyright (C) 2006, 2007 Apple Inc. All rights reserved.
  *           (C) 2008 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  *
  * 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.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 72 matching lines @@
     textBlockStyle->setDisplay(BLOCK);
     textBlockStyle->setUnique();
 
     return textBlockStyle.release();
 }
 
 LayoutObject* LayoutTextControlMultiLine::layoutSpecialExcludedChild(bool relayoutChildren, SubtreeLayoutScope& layoutScope)
 {
     LayoutObject* placeholderLayoutObject = LayoutTextControl::layoutSpecialExcludedChild(relayoutChildren, layoutScope);
     if (!placeholderLayoutObject)
-        return 0;
+        return nullptr;
     if (!placeholderLayoutObject->isBox())
         return placeholderLayoutObject;
     LayoutBox* placeholderBox = toLayoutBox(placeholderLayoutObject);
     placeholderBox->mutableStyleRef().setLogicalWidth(Length(contentLogicalWidth() - placeholderBox->borderAndPaddingLogicalWidth(), Fixed));
     placeholderBox->layoutIfNeeded();
     placeholderBox->setX(borderLeft() + paddingLeft());
     placeholderBox->setY(borderTop() + paddingTop());
     return placeholderLayoutObject;
 }
 
 }