Crash in print spooler when back pressed before content update complted.

We are updating the printed content asynchronously. If the user cancels
printing before the first update is complete the spooler components are
in a destroyed state but we attempt to handle the completed update resulting
in a crash. Now if printing is cancelled we ignore the update result.

bug:18525491

Change-Id: I1ef47a17de19896cba2c7a1dd2bfc205065e0a5a