Remove usage of SingleThreadTaskRunner from DataReductionProxyConfig.

Remove usage of SingleThreadTaskRunner from DataReductionProxyConfig.

With the removal using the DataReductionProxyService class to perform the secure proxy check, there is no longer a need to post tasks to the UI thread.

This effectively makes DataReductionProxyConfig an IO thread only class, and does not need to have pointers to task runners.

BUG=472290
Committed: https://crrev.com/c1fe98e261ba2dea61a291b3f0b4f9afa2f2ddec
Cr-Commit-Position: refs/heads/master@{#327360}

[jeremyim, 2015-04-28 18:20:23]

jeremyim@chromium.org changed reviewers:
+ bengr@chromium.org

[jeremyim, 2015-04-28 18:20:24]

PTAL. =)

[bengr, 2015-04-28 19:45:54]

lgtm. Thanks for doing this!

[jeremyim, 2015-04-28 19:47:12]

The CQ bit was checked by jeremyim@chromium.org

[commit-bot: I haz the power, 2015-04-28 19:47:39]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1052583005/40001

[commit-bot: I haz the power, 2015-04-28 20:41:53]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-04-28 20:43:08]

Patchset 3 (id:??) landed as
https://crrev.com/c1fe98e261ba2dea61a291b3f0b4f9afa2f2ddec
Cr-Commit-Position: refs/heads/master@{#327360}