Revert of Turn VariationsService::GetVariationsServerURL into instance method

Revert of Turn VariationsService::GetVariationsServerURL into instance method (patchset #5 id:140001 of https://codereview.chromium.org/1313213005/ )

Reason for revert:
Gak! The CreateForTesting method can't call the Create method because the latter returns null in non-official builds -- exactly the problem from the original CL :P. I don't understand why this CL passed trybots.

Original issue's description:
> Turn VariationsService::GetVariationsServerURL into instance method
>
> This will allow for followup work that uses |client_| within that
> method.
>
> Reland of https://codereview.chromium.org/1309843005 with the browsertests
> fixed to remove the unsafe accesses to g_browser_process->variations_service().
>
> BUG=516678
>
> Committed: https://crrev.com/dcc53f65eb04018ca60b077a55aa5923ac5e8e9c
> Cr-Commit-Position: refs/heads/master@{#346878}

TBR=asvitkine@chromium.org,bartfab@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=516678
Committed: https://crrev.com/e6074b23a75c0e4bddffb1ce326d2c80f1dd4e3c
Cr-Commit-Position: refs/heads/master@{#346884}

[blundell, 2015-09-02 11:23:23]

Created Revert of Turn VariationsService::GetVariationsServerURL into instance
method

[commit-bot: I haz the power, 2015-09-02 11:23:28]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1306913003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1306913003/1

[commit-bot: I haz the power, 2015-09-02 11:24:04]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-02 11:24:40]

Patchset 1 (id:??) landed as
https://crrev.com/e6074b23a75c0e4bddffb1ce326d2c80f1dd4e3c
Cr-Commit-Position: refs/heads/master@{#346884}