Fix flaky cast_unittests where threads were being joined incorrectly.

media/cast/test/utility/standalone_cast_environment.cc

Index: media/cast/test/utility/standalone_cast_environment.cc
diff --git a/media/cast/test/utility/standalone_cast_environment.cc b/media/cast/test/utility/standalone_cast_environment.cc
index 562079e796481afc070c93e9a5d66b3426d32682..c9dcb0b9caf92c1d4247329c7c129d8c7a8063f4 100644
--- a/media/cast/test/utility/standalone_cast_environment.cc
+++ b/media/cast/test/utility/standalone_cast_environment.cc
@@ -30,11 +30,14 @@ StandaloneCastEnvironment::StandaloneCastEnvironment()
 }
 
 StandaloneCastEnvironment::~StandaloneCastEnvironment() {
-  DCHECK(CalledOnValidThread());
+  CHECK(CalledOnValidThread());
+  CHECK(!main_thread_.IsRunning());
+  CHECK(!audio_thread_.IsRunning());
+  CHECK(!video_thread_.IsRunning());
 }
 
 void StandaloneCastEnvironment::Shutdown() {
-  DCHECK(CalledOnValidThread());
+  CHECK(CalledOnValidThread());
   main_thread_.Stop();
   audio_thread_.Stop();
   video_thread_.Stop();