Use a TestBrowserThreadBundle in the DeviceSettingsTestBase.

Use a TestBrowserThreadBundle in the DeviceSettingsTestBase.

Lots of policy tests inherit from that class to reuse common setup code.
The recent work on cloud policy for extensions wired a new class to some
of the existing ones that posts a cleanup task to the IO thread, and
DeviceSettingsTestBase wasn't mocking that thread; so these tests started
leaking at shutdown.

The TestBrowserThreadBundle sets up the UI and FILE threads that the tests
were expecting as well as the other common threads, which makes these tests
more robust.

TBR=pastarmovj@chromium.org
BUG=None

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=278648

[Joao da Silva, 2014-06-20 01:02:49]

Here's a fix for the leak. I've verified it locally and looks good; I'm also
quite familiar with this code, so I'll TBR to Julian who is away until Monday.

[Joao da Silva, 2014-06-20 01:06:17]

The CQ bit was checked by joaodasilva@chromium.org

[commit-bot: I haz the power, 2014-06-20 01:07:13]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/joaodasilva@chromium.org/348713004/1

[commit-bot: I haz the power, 2014-06-20 08:30:12]

Change committed as 278648