Have VariationsService get version for simulation callback from client

Have VariationsService get version for simulation callback from client

To get the version to use for simulation, VariationsService relies
on //chrome-level functionality on some platforms. This CL abstracts
that functionality through VariationsServiceClient as part of enabling
componentization of VariationsService.

BUG=516682
Committed: https://crrev.com/449471f72daa60e1ac312945ad7f4244c4d0ad65
Cr-Commit-Position: refs/heads/master@{#345574}

[blundell, 2015-08-24 19:03:03]

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

[Alexei Svitkine (slow), 2015-08-24 19:09:56]

lgtm

[blundell, 2015-08-26 08:47:30]

The CQ bit was checked by blundell@chromium.org

[blundell, 2015-08-26 08:47:31]

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/1310253004/#ps160001
(title: "Rebase")

[commit-bot: I haz the power, 2015-08-26 08:47:47]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1310253004/160001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1310253004/160001

[commit-bot: I haz the power, 2015-08-26 09:46:30]

Committed patchset #9 (id:160001)

[commit-bot: I haz the power, 2015-08-26 09:47:37]

Patchset 9 (id:??) landed as
https://crrev.com/449471f72daa60e1ac312945ad7f4244c4d0ad65
Cr-Commit-Position: refs/heads/master@{#345574}