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

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

Index: ppapi/native_client/src/trusted/plugin/plugin.h
diff --git a/ppapi/native_client/src/trusted/plugin/plugin.h b/ppapi/native_client/src/trusted/plugin/plugin.h
index 8684ffa5a550ea04868395ff6260eb3b2756a2b5..74ec1a8f15b830a03eda42bd6185c2b1e5b33993 100644
--- a/ppapi/native_client/src/trusted/plugin/plugin.h
+++ b/ppapi/native_client/src/trusted/plugin/plugin.h
@@ -166,9 +166,11 @@ class Plugin : public pp::Instance {
                          bool* started,
                          ServiceRuntime* service_runtime);
 
+  // This is invoked on the main thread.
   void LoadNexeAndStart(int32_t pp_error,
+                        ServiceRuntime* service_runtime,
                         PP_NaClFileInfo file_info,
-                        ServiceRuntime* service_runtime);
+                        const pp::CompletionCallback& callback);
 
   // Callback used when getting the URL for the .nexe file.  If the URL loading
   // is successful, the file descriptor is opened and can be passed to sel_ldr