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

Source/core/layout/LayoutButton.cpp

 /**
  * Copyright (C) 2005 Apple Computer, Inc.
  *
  * 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,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  * Library General Public License for more details.
  *
  * You should have received a copy of the GNU Library General Public License
  * along with this library; see the file COPYING.LIB.  If not, write to
  * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
  * Boston, MA 02110-1301, USA.
  *
  */
 
 #include "config.h"
 #include "core/layout/LayoutButton.h"
 
 namespace blink {
 
 using namespace HTMLNames;
 
 LayoutButton::LayoutButton(Element* element)
     : LayoutFlexibleBox(element)
-    , m_inner(0)
+    , m_inner(nullptr)
 {
 }
 
 LayoutButton::~LayoutButton()
 {
 }
 
 void LayoutButton::addChild(LayoutObject* newChild, LayoutObject* beforeChild)
 {
     if (!m_inner) {
@@ skipping 66 matching lines @@
     return LayoutFlexibleBox::baselinePosition(baseline, firstLine, direction, linePositionMode);
 }
 
 
 // For compatibility with IE/FF we only clip overflow on input elements.
 bool LayoutButton::hasControlClip() const
 {
     return !isHTMLButtonElement(node());
 }
 } // namespace blink