Fix memory leak in tests.

media/cast/video_sender/external_video_encoder_unittest.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <vector>
 
 #include "base/bind.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 #include "media/base/video_frame.h"
@@ skipping 112 matching lines @@
   task_runner_->RunTasks();
 
   for (int i = 0; i < 6; ++i) {
     capture_time += base::TimeDelta::FromMilliseconds(33);
     test_video_encoder_callback_->SetExpectedResult(false, i + 1, i,
                                                     capture_time);
     EXPECT_TRUE(video_encoder_->EncodeVideoFrame(video_frame_, capture_time,
                                                  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
 }  // namespace media