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 cc843a3cb290a19037a837a3409c68016cae902e..650aa3d59bcab17053ec45d56158d68fbd9e18f4 100644
--- a/ppapi/native_client/src/trusted/plugin/service_runtime.h
+++ b/ppapi/native_client/src/trusted/plugin/service_runtime.h
@@ -194,6 +194,7 @@ class ServiceRuntime {
   // Establish an SrpcClient to the sel_ldr instance and load the nexe.
   // The nexe to be started is passed through |file_info|.
   // Upon completion |callback| is invoked with status code.
+  // This function must be called on the main thread.
   void LoadNexeAndStart(PP_NaClFileInfo file_info,
                         const pp::CompletionCallback& callback);