[PPAPI] It is now valid to store PP_Resource 0 in a ResourceVar.

Making a resource var from PP_Resource 0 results in an empty resource
var, instead of a null var. Fixed handling of NULL Resources in
PluginResourceVar.

BUG=177017

Review URL: https://codereview.chromium.org/26273008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228835 0039d316-1c4b-4281-b951-d872f2087c98

diff --git a/ppapi/proxy/plugin_resource_var.h b/ppapi/proxy/plugin_resource_var.h
index b02718b6..42be66d2 100644
--- a/ppapi/proxy/plugin_resource_var.h
+++ b/ppapi/proxy/plugin_resource_var.h

@@ -31,6 +31,7 @@
   virtual ~PluginResourceVar();
 
  private:
+  // If NULL, this represents the PP_Resource 0.
   scoped_refptr<ppapi::Resource> resource_;
 
   DISALLOW_COPY_AND_ASSIGN(PluginResourceVar);