Revert of Turn VariationsService::GetVariationsServerURL into instance method

Revert of Turn VariationsService::GetVariationsServerURL into instance method (patchset #2 id:20001 of https://codereview.chromium.org/1312423003/ )

Reason for revert:
It turns out that this change is actually not safe as written: g_browser_process->variations_service() returns null in non-official builds unless a default variations server URL is set on the command line. The reason the patch did not fail is because the method being called was (a) not virtual and (b) didn't use any state of the object.

Original issue's description:
> Turn VariationsService::GetVariationsServerURL into instance method
>
> This will allow for followup work that uses |client_| within that
> method.
>
> BUG=516678
> TBR=bartfab
>
> Committed: https://crrev.com/642578a171c7acee3de6b514bf2738fcb0339aac
> Cr-Commit-Position: refs/heads/master@{#346111}

TBR=asvitkine@chromium.org,bartfab@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=516678
Committed: https://crrev.com/aaececbc843b418e0e19b78ba77bba8d7ac84a26
Cr-Commit-Position: refs/heads/master@{#346141}

[blundell, 2015-08-28 14:21:01]

Created Revert of Turn VariationsService::GetVariationsServerURL into instance
method

[commit-bot: I haz the power, 2015-08-28 14:21:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1320503005/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1320503005/1

[commit-bot: I haz the power, 2015-08-28 14:22:30]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-28 14:23:12]

Patchset 1 (id:??) landed as
https://crrev.com/aaececbc843b418e0e19b78ba77bba8d7ac84a26
Cr-Commit-Position: refs/heads/master@{#346141}