Fix flaky cast_unittests where threads were being joined incorrectly.

media/cast/test/utility/standalone_cast_environment.h

 // Copyright 2014 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.
 
 #ifndef MEDIA_CAST_TEST_UTILITY_STANDALONE_CAST_ENVIRONMENT_H_
 #define MEDIA_CAST_TEST_UTILITY_STANDALONE_CAST_ENVIRONMENT_H_
 
 #include "base/threading/thread.h"
 #include "base/threading/thread_checker.h"
 #include "media/cast/cast_environment.h"
 
 namespace media {
 namespace cast {
 
 // A complete CastEnvironment where all task runners are spurned from
 // internally-owned threads.  Uses base::DefaultTickClock as a clock.
+//
+// A user of StandaloneCastEnvironment *must* call Shutdown() on the same thread
+// that constructed the instance before the ref-count reaches zero.
+// http://crbug.com/396480
 class StandaloneCastEnvironment : public CastEnvironment,
                                   public base::ThreadChecker {
  public:
   StandaloneCastEnvironment();
 
   // Stops all threads backing the task runners, blocking the caller until
   // complete.
   void Shutdown();
 
  protected:
   virtual ~StandaloneCastEnvironment();
 
   base::Thread main_thread_;
   base::Thread audio_thread_;
   base::Thread video_thread_;
 
   DISALLOW_COPY_AND_ASSIGN(StandaloneCastEnvironment);
 };
 
 }  // namespace cast
 }  // namespace media
 
 #endif  // MEDIA_CAST_TEST_UTILITY_STANDALONE_CAST_ENVIRONMENT_H_