Load apps on separate threads in service_manager_unittests

Load apps on separate threads in service_manager_unittests

This loads up and destroys applications on their own thread (using
BackgroundServiceLoader) so we can test the full in-process application
lifecycle.

BUG=396300
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=285410

[jamesr, 2014-07-23 22:28:20]

This depends on https://codereview.chromium.org/380413003/ and re-enabled some
of the tests that disabled.

[DaveMoore, 2014-07-24 16:34:54]

lgtm

[jamesr, 2014-07-24 18:14:38]

The CQ bit was checked by jamesr@chromium.org

[commit-bot: I haz the power, 2014-07-24 18:17:18]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/jamesr@chromium.org/415803005/20001

[commit-bot: I haz the power, 2014-07-24 21:28:41]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  linux_chromium_chromeos_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/linux_chromium_chrome...)
  linux_chromium_rel_swarming on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/linux_chromium_rel_sw...)

[commit-bot: I haz the power, 2014-07-24 22:01:18]

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

[commit-bot: I haz the power, 2014-07-24 22:01:19]

Try jobs failed on following builders:
  linux_chromium_rel_swarming on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/linux_chromium_rel_sw...)

[jamesr, 2014-07-24 22:01:58]

The CQ bit was checked by jamesr@chromium.org

[commit-bot: I haz the power, 2014-07-24 22:04:56]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/jamesr@chromium.org/415803005/20001

[commit-bot: I haz the power, 2014-07-24 22:37:33]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  linux_chromium_chromeos_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/linux_chromium_chrome...)
  linux_chromium_rel_swarming on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/linux_chromium_rel_sw...)

[commit-bot: I haz the power, 2014-07-24 23:44:34]

Change committed as 285410