Fix a few warnings that -Wnull-conversion of a future clang will complain about.

ui/gfx/compositor/layer_unittest.cc

 // Copyright (c) 2012 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 "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/file_path.h"
 #include "base/file_util.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/path_service.h"
@@ skipping 347 matching lines @@
 }
 
 class LayerWithDelegateTest : public testing::Test, public CompositorDelegate {
  public:
   LayerWithDelegateTest() : schedule_draw_invoked_(false) {}
   virtual ~LayerWithDelegateTest() {}
 
   // Overridden from testing::Test:
   virtual void SetUp() OVERRIDE {
     ui::SetupTestCompositor();
-    compositor_.reset(new Compositor(this, NULL, gfx::Size(1000, 1000)));
+    compositor_.reset(new Compositor(
+          this, gfx::kNullAcceleratedWidget, gfx::Size(1000, 1000)));
   }
 
   virtual void TearDown() OVERRIDE {
   }
 
   Compositor* compositor() { return compositor_.get(); }
 
   virtual Layer* CreateLayer(LayerType type) {
     return new Layer(type);
   }
@@ skipping 635 matching lines @@
   EXPECT_TRUE(schedule_draw_invoked_);
   // Because SchedulePaint() was invoked from OnPaintLayer() |child| should
   // still need to be painted.
   DrawTree(root.get());
   EXPECT_EQ(1, child_delegate.GetPaintCountAndClear());
   EXPECT_TRUE(child_delegate.last_clip_rect().Contains(
                   gfx::Rect(10, 10, 30, 30)));
 }
 
 } // namespace ui