Fix flaky cast_unittests where threads were being joined incorrectly.

media/cast/test/utility/standalone_cast_environment.h

Index: media/cast/test/utility/standalone_cast_environment.h
diff --git a/media/cast/test/utility/standalone_cast_environment.h b/media/cast/test/utility/standalone_cast_environment.h
index 262f7d6e577d2e5150cb4483c5a6df8e5e2c8c56..91240e1ce55bef79e1b432bccbc47c5ed0fc81ef 100644
--- a/media/cast/test/utility/standalone_cast_environment.h
+++ b/media/cast/test/utility/standalone_cast_environment.h
@@ -14,6 +14,10 @@ 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: