GitLab

- IBusEngineFactoryService takes the ownership of engine_id - CreateEngine handler mapping.
- Make IBusEngineFactoryService::CreateEngine asynchronous, and introduce asynchronous unittest.

BUG=None
TEST=ran chromeos_unittests


Review URL: https://chromiumcodereview.appspot.com/10836047

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@149579 0039d316-1c4b-4281-b951-d872f2087c98