app_list: Fix UpdateIcon crash for app update.
When an app is unloaded for updating, UpdateIcon() is called. However,
GetExtension() returns NULL at this time and triggers a crash in HasOverlay().
Since HasOverlay() info is not changing, cache it in a |has_overlay_| to fix
the crash.
Also fix a potential stale title/icon problem when extension app is updated.
BUG=157996,169463
TEST=Verify app list should not cause a crash when an app is updated.
R=sky@chromium.org,benwells@chromium.org
Review URL: https://chromiumcodereview.appspot.com/
11885006
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@176584 0039d316-1c4b-4281-b951-d872f2087c98