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

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

 /* -*- c++ -*- */
 /*
  * Copyright (c) 2012 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 // A class containing information regarding a socket connection to a
 // service runtime instance.
 
@@ skipping 176 matching lines @@
   // actually start.
   void WaitForNexeStart();
 
   // Signal to waiting threads that LoadNexeAndStart is complete (either
   // successfully or unsuccessfully).
   void SignalNexeStarted();
 
   // 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);
 
   // Starts the application channel to the nexe.
   SrpcClient* SetupAppChannel();
 
   bool Log(int severity, const nacl::string& msg);
   Plugin* plugin() const { return plugin_; }
   void Shutdown();
 
@@ skipping 36 matching lines @@
   // Mutex and CondVar to protect start_sel_ldr_done_ and nexe_started_.
   NaClMutex mu_;
   NaClCondVar cond_;
   bool start_sel_ldr_done_;
   bool nexe_started_;
 };
 
 }  // namespace plugin
 
 #endif  // NATIVE_CLIENT_SRC_TRUSTED_PLUGIN_SERVICE_RUNTIME_H_