[Ozone-DRI] Remove the extra helper thread and reuse the IO one

[Ozone-DRI] Remove the extra helper thread and reuse the IO one

Now that we have access to the IO thread, pass the IO task runner to
the main thread and use that to register and listen for page flip
events.

BUG=none

Committed: https://crrev.com/1e5b1c35d814e721544bddea9b0de06eaf856f5a
Cr-Commit-Position: refs/heads/master@{#314341}

[dnicoara, 2015-02-02 19:34:24]

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

[dnicoara, 2015-02-02 19:34:24]

PTAL

[spang, 2015-02-02 20:50:38]

lgtm

[dnicoara, 2015-02-03 14:55:34]

The CQ bit was checked by dnicoara@chromium.org

[commit-bot: I haz the power, 2015-02-03 14:56:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/891703006/20001

[commit-bot: I haz the power, 2015-02-03 16:11:50]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-02-03 16:13:00]

Patchset 2 (id:??) landed as
https://crrev.com/1e5b1c35d814e721544bddea9b0de06eaf856f5a
Cr-Commit-Position: refs/heads/master@{#314341}