Fix flaky cast_unittests where threads were being joined incorrectly.

media/cast/receiver/video_decoder_unittest.cc

Index: media/cast/receiver/video_decoder_unittest.cc
diff --git a/media/cast/receiver/video_decoder_unittest.cc b/media/cast/receiver/video_decoder_unittest.cc
index c7c39ad2d06b1fc71e6cd9fe92b1676cc78ecfe2..7ea4b5da141d19e22a0de74345cd97575a284a71 100644
--- a/media/cast/receiver/video_decoder_unittest.cc
+++ b/media/cast/receiver/video_decoder_unittest.cc
@@ -44,6 +44,11 @@ class VideoDecoderTest : public ::testing::TestWithParam<Codec> {
     vp8_encoder_.Initialize();
   }
 
+  virtual ~VideoDecoderTest() {
+    // Make sure all threads have stopped before the environment goes away.
+    cast_environment_->Shutdown();
+  }
+
  protected:
   virtual void SetUp() OVERRIDE {
     video_decoder_.reset(new VideoDecoder(cast_environment_, GetParam()));