Add limitation that LoadNexeAndStart must be called on the main thread.

ppapi/native_client/src/trusted/plugin/service_runtime.h

Index: ppapi/native_client/src/trusted/plugin/service_runtime.h
diff --git a/ppapi/native_client/src/trusted/plugin/service_runtime.h b/ppapi/native_client/src/trusted/plugin/service_runtime.h
index bc5f24f9ddb79cb3ccc506799128b374535f76ed..852379ed6d9a40cebd8f9c3738bc53304ac85109 100644
--- a/ppapi/native_client/src/trusted/plugin/service_runtime.h
+++ b/ppapi/native_client/src/trusted/plugin/service_runtime.h
@@ -193,9 +193,9 @@ class ServiceRuntime {
 
   // Establish an SrpcClient to the sel_ldr instance and load the nexe.
   // The nexe to be started is passed through |file_info|.
+  // This function must be called on the main thread.
   void LoadNexeAndStart(PP_NaClFileInfo file_info,
-                        const pp::CompletionCallback& started_cb,
-                        const pp::CompletionCallback& crash_cb);
+                        const pp::CompletionCallback& callback);
 
   // Starts the application channel to the nexe.
   SrpcClient* SetupAppChannel();