Fix at_exit error in video_decode tests

Fix at_exit error in video_decode tests

ui::OzoneGpuTestHelper needs to be uninstantiated in TearDown() since it
creates a base::Thread which wants to register with the AtExitManager.
However the AtExitManager runs out of scope before the
testing::Environment.

BUG=447798
TEST=compile and run ./video_decode_accelerator_unittest (I had
https://codereview.chromium.org/938873002/ applied as well to make sure
tests succeed)

Committed: https://crrev.com/c7a207e106d84ac81344a7c7cef8394ed672f0a3
Cr-Commit-Position: refs/heads/master@{#319148}

[dnicoara, 2015-02-24 17:44:15]

dnicoara@chromium.org changed reviewers:
+ posciak@chromium.org

[dnicoara, 2015-02-24 17:44:15]

Please take a look. Sorry for the OzoneGpuTestHelper omission.

[dnicoara, 2015-03-03 15:37:56]

Gentle ping.

[Pawel Osciak, 2015-03-04 01:16:58]

Sorry missed this. lgtm.

[dnicoara, 2015-03-04 02:09:42]

The CQ bit was checked by dnicoara@chromium.org

[commit-bot: I haz the power, 2015-03-04 02:11:06]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/949183004/40001

[commit-bot: I haz the power, 2015-03-04 05:13:07]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-03-04 05:13:08]

Try jobs failed on following builders:
  win_chromium_compile_dbg_ng on tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_comp...)

[dnicoara, 2015-03-04 13:33:22]

The CQ bit was checked by dnicoara@chromium.org

[commit-bot: I haz the power, 2015-03-04 13:41:30]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/949183004/40001

[commit-bot: I haz the power, 2015-03-04 16:51:27]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-03-04 16:51:28]

Try jobs failed on following builders:
  win_chromium_compile_dbg_ng on tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_comp...)

[dnicoara, 2015-03-04 16:53:54]

The CQ bit was checked by dnicoara@chromium.org

[commit-bot: I haz the power, 2015-03-04 16:54:28]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/949183004/40001

[commit-bot: I haz the power, 2015-03-04 20:00:25]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-03-04 20:00:26]

Try jobs failed on following builders:
  win_chromium_compile_dbg_ng on tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_comp...)

[dnicoara, 2015-03-04 22:31:25]

The CQ bit was checked by dnicoara@chromium.org

[dnicoara, 2015-03-04 22:31:25]

The patchset sent to the CQ was uploaded after l-g-t-m from posciak@chromium.org
Link to the patchset: https://codereview.chromium.org/949183004/#ps60001 (title:
"rebase")

[commit-bot: I haz the power, 2015-03-04 22:42:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/949183004/60001

[commit-bot: I haz the power, 2015-03-04 22:47:40]

Committed patchset #4 (id:60001)

[commit-bot: I haz the power, 2015-03-04 22:48:22]

Patchset 4 (id:??) landed as
https://crrev.com/c7a207e106d84ac81344a7c7cef8394ed672f0a3
Cr-Commit-Position: refs/heads/master@{#319148}