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You can't Quit() the MessageLoop of a base::Thread. This CL makes
BackgroundServiceLoader use DelegateSimpleThread and run a MessageLoop
instead of using base::Thread, so that applications can safely Quit()
their current MessageLoop as usual to signify that they are done.

This shouldn't be necessary for the desktop shell once services are moved
out (network, NVS), but we'll likely continue to use this loader on android.

Note: the quit_on_shutdown() method is being removed in https://codereview.chromium.org/394903005/

BUG=

Review URL: https://codereview.chromium.org/437493002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@287475 0039d316-1c4b-4281-b951-d872f2087c98