Allow SiteEngagementService::IsEnabled to respect field trial groups.

Allow SiteEngagementService::IsEnabled to respect field trial groups.

This CL allows the site engagement service to be activated when a
field trial group name starting with "Enabled" is present.

BUG=539723
Committed: https://crrev.com/f823ead71e30e5b79cd0cda41a7d5bb8366973e0
Cr-Commit-Position: refs/heads/master@{#352772}

[dominickn, 2015-10-07 04:05:33]

dominickn@chromium.org changed reviewers:
+ calamity@chromium.org

[dominickn, 2015-10-07 04:05:33]

PTAL, thanks

[calamity, 2015-10-07 04:27:22]

lgtm

[dominickn, 2015-10-07 04:33:07]

The CQ bit was checked by dominickn@chromium.org

[commit-bot: I haz the power, 2015-10-07 04:33:21]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1393723002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1393723002/20001

[commit-bot: I haz the power, 2015-10-07 04:39:13]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-10-07 04:40:20]

Patchset 2 (id:??) landed as
https://crrev.com/f823ead71e30e5b79cd0cda41a7d5bb8366973e0
Cr-Commit-Position: refs/heads/master@{#352772}