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

content/browser/renderer_host/test_render_view_host.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 "content/browser/renderer_host/test_backing_store.h"
 #include "content/browser/renderer_host/test_render_view_host.h"
 #include "content/browser/site_instance_impl.h"
 #include "content/browser/tab_contents/navigation_controller_impl.h"
 #include "content/browser/tab_contents/test_web_contents.h"
 #include "content/common/dom_storage_common.h"
@@ skipping 118 matching lines @@
 
 bool TestRenderWidgetHostView::PostProcessEventForPluginIme(
     const NativeWebKeyboardEvent& event) {
   return false;
 }
 
 gfx::PluginWindowHandle
 TestRenderWidgetHostView::AllocateFakePluginWindowHandle(
     bool opaque,
     bool root) {
-  return NULL;
+  return gfx::kNullPluginWindow;
 }
 
 void TestRenderWidgetHostView::DestroyFakePluginWindowHandle(
     gfx::PluginWindowHandle window) {
 }
 
 void TestRenderWidgetHostView::AcceleratedSurfaceSetIOSurface(
     gfx::PluginWindowHandle window,
     int32 width,
     int32 height,
@@ skipping 163 matching lines @@
 
 TestRenderViewHost* RenderViewHostImplTestHarness::active_test_rvh() {
   return static_cast<TestRenderViewHost*>(active_rvh());
 }
 
 TestWebContents* RenderViewHostImplTestHarness::contents() {
   return static_cast<TestWebContents*>(web_contents());
 }
 
 }  // namespace content