Add a message loop to TabControllerTest

Add a message loop to TabControllerTest

This makes it possible to later migrate base/ from MessageLoopProxy
over to ThreadTaskRunnerHandle, which requires an active task runner on
the current thread.

TBR=msw@chromium.org
BUG=465354
Committed: https://crrev.com/4265822a06571e85a26c659ea93186243c8f8e10
Cr-Commit-Position: refs/heads/master@{#327496}

[Sami, 2015-04-29 15:31:30]

The CQ bit was checked by skyostil@chromium.org

[commit-bot: I haz the power, 2015-04-29 15:32:15]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1114993002/1

[commit-bot: I haz the power, 2015-04-29 15:42:35]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-29 15:43:31]

Patchset 1 (id:??) landed as
https://crrev.com/4265822a06571e85a26c659ea93186243c8f8e10
Cr-Commit-Position: refs/heads/master@{#327496}

[msw, 2015-04-29 17:21:05]

Why does this test need a MessageLoop? I don't see any calls to post tasks in
that file itself. Presumably something it uses requires a MessageLoop? It'd be
better to TBR someone that actually knew about this test or your message loop
effort.

[Sami, 2015-04-29 17:34:53]

On 2015/04/29 17:21:05, msw wrote:
> Why does this test need a MessageLoop? I don't see any calls to post tasks in
> that file itself. Presumably something it uses requires a MessageLoop? It'd be
> better to TBR someone that actually knew about this test or your message loop
> effort.

Sorry for not giving more context. The basic problem is that there is an
indirect dependency from this test to functionality in base/ (e.g.,
SequencedTaskRunner) which will start requiring an active task runner on the
current thread. In https://codereview.chromium.org/1075213002/ we decided to
make it the responsibility of each test to ensure they have the necessary
environment set up for the feature they are testing. This is both more realistic
and safer because we can remove some test-only code paths from production code.

I should've included this particular fix in
https://codereview.chromium.org/1086733002/ but accidentally left it out.

[msw, 2015-04-29 17:36:23]

rubber stamp lgtm