Expose API on VariationsService to set the restrict mode param.

Expose API on VariationsService to set the restrict mode param.

This paves the way to allowing logic living elsewhere in Chrome to
specify a value for this param (which will come in a later CL).

Also, cleans up a couple areas of the code, including cleaning up the
unit test for the URL generating function to abstract away the policy
pref code and using a weak pointer instead of base::Unretained() in
a callback.

BUG=452605
Committed: https://crrev.com/96339279b60df2f9811b0d56d4d435f1069c4e50
Cr-Commit-Position: refs/heads/master@{#313504}

[Alexei Svitkine (slow), 2015-01-27 19:24:38]

Patchset #2 (id:20001) has been deleted

[Alexei Svitkine (slow), 2015-01-27 19:24:44]

Patchset #1 (id:1) has been deleted

[Alexei Svitkine (slow), 2015-01-27 19:24:49]

Patchset #1 (id:40001) has been deleted

[Alexei Svitkine (slow), 2015-01-27 19:51:02]

Patchset #1 (id:60001) has been deleted

[Alexei Svitkine (slow), 2015-01-27 19:51:07]

Patchset #1 (id:80001) has been deleted

[Alexei Svitkine (slow), 2015-01-27 19:51:40]

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

[jwd, 2015-01-27 22:47:19]

lgtm

[Alexei Svitkine (slow), 2015-01-27 22:52:23]

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

[Alexei Svitkine (slow), 2015-01-27 22:52:23]

+mnissler for policy/ owners

[Mattias Nissler (ping if slow), 2015-01-28 09:07:16]

policy/ LGTM

Feel free to TBR trivial adjustments after API changes have reviewed by
respective owners.

[Alexei Svitkine (slow), 2015-01-28 14:23:39]

The CQ bit was checked by asvitkine@chromium.org

[commit-bot: I haz the power, 2015-01-28 14:24:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/883803002/100001

[commit-bot: I haz the power, 2015-01-28 14:32:49]

Committed patchset #1 (id:100001)

[commit-bot: I haz the power, 2015-01-28 14:33:41]

Patchset 1 (id:??) landed as
https://crrev.com/96339279b60df2f9811b0d56d4d435f1069c4e50
Cr-Commit-Position: refs/heads/master@{#313504}