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

webkit/plugins/npapi/webplugin_delegate_impl_mac.mm

 // Copyright (c) 2011 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.
 
 #import <Cocoa/Cocoa.h>
 #import <QuartzCore/QuartzCore.h>
 
 #include "webkit/plugins/npapi/webplugin_delegate_impl.h"
 
 #include <string>
@@ skipping 238 matching lines @@
          WebInputEvent::RightButtonDown |
          WebInputEvent::MiddleButtonDown;
 }
 
 #pragma mark -
 #pragma mark Core WebPluginDelegate implementation
 
 WebPluginDelegateImpl::WebPluginDelegateImpl(
     gfx::PluginWindowHandle containing_view,
     PluginInstance *instance)
-    : windowed_handle_(NULL),
+    : windowed_handle_(gfx::kNullPluginWindow),
       // all Mac plugins are "windowless" in the Windows/X11 sense
       windowless_(true),
       plugin_(NULL),
       instance_(instance),
       parent_(containing_view),
       quirks_(0),
       use_buffer_context_(true),
       buffer_context_(NULL),
       layer_(nil),
       surface_(NULL),
@@ skipping 149 matching lines @@
     default:
       NOTREACHED();
       break;
   }
 
   // Let the WebPlugin know that we are windowless, unless this is a Core
   // Animation plugin, in which case AcceleratedPluginEnabledRendering
   // calls SetWindow. Rendering breaks if SetWindow is called before
   // accelerated rendering is enabled.
   if (!layer_)
-    plugin_->SetWindow(NULL);
+    plugin_->SetWindow(gfx::kNullPluginWindow);
 
 #ifndef NP_NO_CARBON
   // If the plugin wants Carbon events, hook up to the source of idle events.
   if (instance()->event_model() == NPEventModelCarbon)
     UpdateIdleEventRate();
 #endif
 
   // QuickTime (in QD mode only) can crash if it gets other calls (e.g.,
   // NPP_Write) before it gets a SetWindow call, so call SetWindow (with a 0x0
   // rect) immediately.
@@ skipping 799 matching lines @@
   qd_manager_->SetFastPathEnabled(enabled);
   qd_port_.port = qd_manager_->port();
   WindowlessSetWindow();
   // Send a paint event so that the new buffer gets updated immediately.
   WindowlessPaint(buffer_context_, clip_rect_);
 }
 #endif  // !NP_NO_QUICKDRAW
 
 }  // namespace npapi
 }  // namespace webkit