Don't DCHECK in AttachmentServiceProxyForTest when MessageLoop is absent

Don't DCHECK in AttachmentServiceProxyForTest when MessageLoop is absent

The code in AttachmentServiceProxyForTest::Create checks that
base::ThreadTaskRunnerHandle::Get() returns valid runner in the
incorrect way. Get() DCHECKs that there is valid MessageLoop.
Instead it should check ThreadTaskRunnerHandle::IsSet().

BUG=449327
R=maniscalco@chromium.org

Committed: https://crrev.com/a8e369960cfab093ffffc9dc9f0066cca2b56812
Cr-Commit-Position: refs/heads/master@{#311917}

[pavely, 2015-01-16 01:08:30]

PTAL.

[maniscalco, 2015-01-16 01:27:33]

On 2015/01/16 01:08:30, pavely wrote:
> PTAL.

LGTM

[pavely, 2015-01-16 17:16:27]

The CQ bit was checked by pavely@chromium.org

[commit-bot: I haz the power, 2015-01-16 17:17:09]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/814273006/1

[commit-bot: I haz the power, 2015-01-16 17:21:13]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-16 17:22:11]

Patchset 1 (id:??) landed as
https://crrev.com/a8e369960cfab093ffffc9dc9f0066cca2b56812
Cr-Commit-Position: refs/heads/master@{#311917}