browser-plugin: Set that we need XEmbed

This makes the plugin work under WebKit-based browsers such as Chromium and
Epiphany. See http://code.google.com/p/chromium/issues/detail?id=38229 and
WindowedCreatePlugin() in
http://src.chromium.org/viewvc/chrome/trunk/src/webkit/plugins/npapi/webplugin_delegate_impl_gtk.cc?revision=86823&content-type=text%2Fplain
for more information.

https://bugzilla.gnome.org/show_bug.cgi?id=663823

browser-plugin/browser-plugin.c

@@ -816,6 +816,11 @@ NPP_GetValue(NPP          instance,
 
     *(NPObject**)value = funcs.createobject (instance, &plugin_class);
     break;
+
+  case NPPVpluginNeedsXEmbed:
+    *(bool *)value = TRUE;
+    break;
+
   default:
     ;
   }