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

Source/core/layout/MultiColumnFragmentainerGroupTest.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 
 #include "core/layout/MultiColumnFragmentainerGroup.h"
 
 #include "core/layout/LayoutMultiColumnFlowThread.h"
 #include "core/layout/LayoutMultiColumnSet.h"
 #include "core/layout/LayoutTestHelper.h"
 
 #include <gtest/gtest.h>
 
 namespace blink {
 
 namespace {
 
 class MultiColumnFragmentainerGroupTest : public RenderingTest {
 public:
-    MultiColumnFragmentainerGroupTest() : m_flowThread(0), m_columnSet(0) { }
+    MultiColumnFragmentainerGroupTest() : m_flowThread(nullptr), m_columnSet(nullptr) { }
 
 protected:
     virtual void SetUp() override;
     virtual void TearDown() override;
 
     LayoutMultiColumnSet& columnSet() { return *m_columnSet; }
 
     static int groupCount(const MultiColumnFragmentainerGroupList&);
 
 private:
@@ skipping 66 matching lines @@
     EXPECT_EQ(groupCount(groupList), 3);
     groupList.addExtraGroup();
     EXPECT_EQ(groupCount(groupList), 4);
     groupList.deleteExtraGroups();
     EXPECT_EQ(groupCount(groupList), 1);
 }
 
 } // anonymous namespace
 
 } // blink