Groom user manager's code.

Groom user manager's code.

- Adds missing files in BUILD.gn
- path/to/grit/... would be more preferable than grit/...
- remove unnecessary consts

BUG=None
R=nkostylev@chromium.org
TBR=oshima@chromium.org
TEST=manually

Committed: https://crrev.com/9fa7761f99e2a1e9605cecd45bff5bf48b5f6823
Cr-Commit-Position: refs/heads/master@{#299023}

[Nikita (slow), 2014-10-09 11:02:26]

lgtm

[Jun Mukai, 2014-10-09 19:02:53]

The CQ bit was checked by mukai@chromium.org

[commit-bot: I haz the power, 2014-10-09 19:03:29]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/644613002/1

[commit-bot: I haz the power, 2014-10-09 19:13:35]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-10-09 19:13:36]

Try jobs failed on following builders:
  chromium_presubmit on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/chromium_presub...)

[Jun Mukai, 2014-10-09 23:36:00]

TBR=oshima for tiny DEPS change

[Jun Mukai, 2014-10-09 23:36:04]

The CQ bit was checked by mukai@chromium.org

[commit-bot: I haz the power, 2014-10-09 23:38:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/644613002/1

[commit-bot: I haz the power, 2014-10-10 00:23:35]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-10 00:24:38]

Patchset 1 (id:??) landed as
https://crrev.com/9fa7761f99e2a1e9605cecd45bff5bf48b5f6823
Cr-Commit-Position: refs/heads/master@{#299023}