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

webkit/tools/test_shell/mac/test_webview_delegate.mm

 // 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 "webkit/tools/test_shell/test_webview_delegate.h"
 
 #import <Cocoa/Cocoa.h>
 #include "base/sys_string_conversions.h"
 #include "base/utf_string_conversions.h"
 #include "third_party/WebKit/Source/WebKit/chromium/public/WebCursorInfo.h"
@@ skipping 215 matching lines @@
 
 // WebPluginPageDelegate ------------------------------------------------------
 
 webkit::npapi::WebPluginDelegate* TestWebViewDelegate::CreatePluginDelegate(
     const FilePath& path,
     const std::string& mime_type) {
   WebWidgetHost *host = GetWidgetHost();
   if (!host)
     return NULL;
 
-  gfx::PluginWindowHandle containing_view = NULL;
+  gfx::PluginWindowHandle containing_view = gfx::kNullPluginWindow;
   WebPluginDelegateImpl* delegate =
       WebPluginDelegateImpl::Create(path, mime_type, containing_view);
   if (delegate)
     delegate->SetNoBufferContext();
   return delegate;
 }
 
 void TestWebViewDelegate::CreatedPluginWindow(
     gfx::PluginWindowHandle handle) {
 }
@@ skipping 29 matching lines @@
 void TestWebViewDelegate::SetPageTitle(const string16& title) {
   [[shell_->webViewHost()->view_handle() window]
       setTitle:[NSString stringWithUTF8String:UTF16ToUTF8(title).c_str()]];
 }
 
 void TestWebViewDelegate::SetAddressBarURL(const GURL& url) {
   const char* frameURL = url.spec().c_str();
   NSString *address = [NSString stringWithUTF8String:frameURL];
   [shell_->editWnd() setStringValue:address];
 }