Fix race condition on kiosk app launch.

The previous code would fail for the case of the app window already
present before the observer gets registered.

BUG=chromium:343540

Review URL: https://codereview.chromium.org/163493002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251088 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251088