Fix flaky cast_unittests where threads were being joined incorrectly.

media/cast/receiver/audio_decoder_unittest.cc

Index: media/cast/receiver/audio_decoder_unittest.cc
diff --git a/media/cast/receiver/audio_decoder_unittest.cc b/media/cast/receiver/audio_decoder_unittest.cc
index 576e9dbce57ac67fb1cfe339a715ed4d5e679a4f..62e4fbbf616e0afc9485ab25a7480e1fbfbb86c3 100644
--- a/media/cast/receiver/audio_decoder_unittest.cc
+++ b/media/cast/receiver/audio_decoder_unittest.cc
@@ -35,6 +35,11 @@ class AudioDecoderTest : public ::testing::TestWithParam<TestScenario> {
       : cast_environment_(new StandaloneCastEnvironment()),
         cond_(&lock_) {}
 
+  virtual ~AudioDecoderTest() {
+    // Make sure all threads have stopped before the environment goes away.
+    cast_environment_->Shutdown();
+  }
+
  protected:
   virtual void SetUp() OVERRIDE {
     audio_decoder_.reset(new AudioDecoder(cast_environment_,