Fix memory leak in tests.

media/cast/video_sender/external_video_encoder_unittest.cc

Index: media/cast/video_sender/external_video_encoder_unittest.cc
diff --git a/media/cast/video_sender/external_video_encoder_unittest.cc b/media/cast/video_sender/external_video_encoder_unittest.cc
index b159280c805c3bdb5e3084e9e8e5135d0cb458a2..f44d17799b44e7e451d85be2c2c781e2f9618994 100644
--- a/media/cast/video_sender/external_video_encoder_unittest.cc
+++ b/media/cast/video_sender/external_video_encoder_unittest.cc
@@ -130,6 +130,9 @@ TEST_F(ExternalVideoEncoderTest, EncodePattern30fpsRunningOutOfAck) {
                                                  frame_encoded_callback));
     task_runner_->RunTasks();
   }
+  // We need to run the task to cleanup the GPU instance.
+  video_encoder_.reset(NULL);
+  task_runner_->RunTasks();
 }
 
 }  // namespace cast