Eliminate implicit conversion from scoped_refptr<T> to T*

ppapi/proxy/plugin_globals.cc

Index: ppapi/proxy/plugin_globals.cc
diff --git a/ppapi/proxy/plugin_globals.cc b/ppapi/proxy/plugin_globals.cc
index 360f2215270ea3da7055b9bb7d92d9dc5e513988..bc37022c8cc6d956dfd8cf514544a41962632001 100644
--- a/ppapi/proxy/plugin_globals.cc
+++ b/ppapi/proxy/plugin_globals.cc
@@ -39,11 +39,11 @@ VarTracker* PluginGlobals::GetVarTracker() {
   return &plugin_var_tracker_;
 }
 
-CallbackTracker* PluginGlobals::GetCallbackTrackerForInstance(
+scoped_refptr<CallbackTracker> PluginGlobals::GetCallbackTrackerForInstance(
     PP_Instance instance) {
   // In the plugin process, the callback tracker is always the same, regardless
   // of the instance.
-  return callback_tracker_.get();
+  return callback_tracker_;
 }
 
 FunctionGroupBase* PluginGlobals::GetFunctionAPI(PP_Instance inst, ApiID id) {