cros: Fix unit_tests crashes in GetBrand() on official builders

cros: Fix unit_tests crashes in GetBrand() on official builders

unit_tests do not have prefs, but branded builds on Chrome OS attempt to read
the brand code out of a null g_browser_process->local_state().

Instead, we now return an empty brand code in tests.

This reduces the number of crashing unit_tests on the official builders from
~300 to ~30.

BUG=430266
Committed: https://crrev.com/8747d18fb89ca9470a6e66caca6607a0c02f0682
Cr-Commit-Position: refs/heads/master@{#302733}

[Ilya Sherman, 2014-11-05 00:33:38]

isherman@chromium.org changed reviewers:
+ isherman@chromium.org

[Ilya Sherman, 2014-11-05 00:33:39]

LGTM

[James Cook, 2014-11-05 00:34:55]

The CQ bit was checked by jamescook@chromium.org

[commit-bot: I haz the power, 2014-11-05 00:36:34]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/703623005/1

[commit-bot: I haz the power, 2014-11-05 01:29:18]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-11-05 01:30:46]

Patchset 1 (id:??) landed as
https://crrev.com/8747d18fb89ca9470a6e66caca6607a0c02f0682
Cr-Commit-Position: refs/heads/master@{#302733}