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

webkit/plugins/npapi/webplugin_delegate_impl_mac.mm

Index: webkit/plugins/npapi/webplugin_delegate_impl_mac.mm
diff --git a/webkit/plugins/npapi/webplugin_delegate_impl_mac.mm b/webkit/plugins/npapi/webplugin_delegate_impl_mac.mm
index 5127eafdd7398aa09cc71289f9448df1b082503c..8a24c77ee07a03b285612618a24a84be1720109c 100644
--- a/webkit/plugins/npapi/webplugin_delegate_impl_mac.mm
+++ b/webkit/plugins/npapi/webplugin_delegate_impl_mac.mm
@@ -256,7 +256,7 @@ int ExternalDragTracker::WebEventButtonModifierMask() {
 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),
@@ -426,7 +426,7 @@ bool WebPluginDelegateImpl::PlatformInitialize() {
   // 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.