Fix memory leak in tests.

media/cast/video_sender/video_sender_unittest.cc

Index: media/cast/video_sender/video_sender_unittest.cc
diff --git a/media/cast/video_sender/video_sender_unittest.cc b/media/cast/video_sender/video_sender_unittest.cc
index 0f0852df6ce83fe6f310a5daa1550d8ba9085535..42c25d2dfd1a2a263b4b577cf59577c53f2b8347 100644
--- a/media/cast/video_sender/video_sender_unittest.cc
+++ b/media/cast/video_sender/video_sender_unittest.cc
@@ -129,6 +129,10 @@ TEST_F(VideoSenderTest, ExternalEncoder) {
   video_sender_->InsertRawVideoFrame(video_frame, capture_time);
 
   task_runner_->RunTasks();
+
+  // We need to run the task to cleanup the GPU instance.
+  video_sender_.reset(NULL);
+  task_runner_->RunTasks();
 }
 
 TEST_F(VideoSenderTest, RtcpTimer) {