Eliminate content::BrowserThread knowledge from VariationsService

Eliminate content::BrowserThread knowledge from VariationsService

Part of enabling VariationsService to be cleanly shared with iOS, which does
not use //content.

BUG=516678
Committed: https://crrev.com/866f75c5ee70a7905e3effc14176c097e561d2eb
Cr-Commit-Position: refs/heads/master@{#345313}

[blundell, 2015-08-19 14:08:27]

blundell@chromium.org changed reviewers:
+ asvitkine@chromium.org

[Alexei Svitkine (slow), 2015-08-24 14:26:16]

lgtm

[blundell, 2015-08-25 12:43:05]

The CQ bit was checked by blundell@chromium.org

[blundell, 2015-08-25 12:43:05]

The patchset sent to the CQ was uploaded after l-g-t-m from
asvitkine@chromium.org
Link to the patchset: https://codereview.chromium.org/1301903005/#ps40001
(title: "Rebase")

[commit-bot: I haz the power, 2015-08-25 12:43:16]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1301903005/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1301903005/40001

[commit-bot: I haz the power, 2015-08-25 13:13:46]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-08-25 13:14:25]

Patchset 3 (id:??) landed as
https://crrev.com/866f75c5ee70a7905e3effc14176c097e561d2eb
Cr-Commit-Position: refs/heads/master@{#345313}