Don't consider shutdown acknowledgement for external extensions

Don't consider shutdown acknowledgement for external extensions

External extensions display a warning, and, on shutdown, we close that warning
and treat it as an "abort". This has the effect of uninstalling the extension -
this is bad, because a) the user won't expect it, and b) we're in the process of
shutting down and shouldn't be doing something like uninstalling. Only treat an
abort as an uninstall if it's user initiated.
BUG=370222
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=268947

[Yoyo Zhou, 2014-05-06 17:55:11]

LGTM

[Devlin, 2014-05-07 15:49:58]

The CQ bit was checked by rdevlin.cronin@chromium.org

[commit-bot: I haz the power, 2014-05-07 15:52:33]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/rdevlin.cronin@chromium.org/261713003/1

[commit-bot: I haz the power, 2014-05-07 21:09:49]

Change committed as 268947