Merge r255817 and r257069: Do not initialize volume manager for signin profiles.

Merge r255817 and r257069: Do not initialize volume manager for signin profiles.

The first one fixies the buggy behavior.
> Do not initialize the volume manager for sign in profiles.
> 
> Since the volumes are not used, and moreover some of them are shared
> between profiles, they volumes should not be mounted, and observers
> shouldn't be added.
> 
> TEST=Tested manually on a enterprise enrolled device.
> BUG=349298
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=255817

The second one is to address regressions due to the code depending on the above
mentioned buggy behavior.
> Accept the DISK_CHANGED event and force mounting.
> 
> Before, the DISK_CHANGED event was ignored. However, it is required, to mount
> disks which are not mounted, because mounting has been discarded because of
> either: happened during sign-in, profile with kExternalStorageDisabled = true,
> or before or during boot.
> 
> TEST=Partly browser_tests. Tested manually on a enterprise enrolled chromebook.
> BUG=349298
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=257069

TBR=mtomasz@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=258188

[kinaba, 2014-03-20 01:56:05]

Committed patchset #1 manually as r258188 (tree was closed).