Fix refreshing the volume list when the volume is unmounted.

Fix refreshing the volume list when the volume is unmounted.

When a volume is unmounted, while it was selected, then the list model is
updated, and since there is no selection anymore, the current entry from the
directory model is selected. However, the corresponding VolumeInfo is gone
so we can't call resolveRootEntry().

This patch adds a check. If the volume is gone, then the selection is aborted.

TEST=Tested manually with FSP.
BUG=418513
Committed: https://crrev.com/4b84385dafb109ba1cd8e2f78366d3585ddbe059
Cr-Commit-Position: refs/heads/master@{#297188}

[mtomasz, 2014-09-29 11:26:10]

mtomasz@chromium.org changed reviewers:
+ fukino@chromium.org

[mtomasz, 2014-09-29 11:26:11]

@fukino: PTAL at this tiny patch. Thanks!

[fukino, 2014-09-29 16:08:13]

On 2014/09/29 11:26:11, mtomasz wrote:
> @fukino: PTAL at this tiny patch. Thanks!

lgtm. Thank you for fixing this!

[mtomasz, 2014-09-29 16:19:13]

On 2014/09/29 16:08:13, fukino wrote:
> On 2014/09/29 11:26:11, mtomasz wrote:
> > @fukino: PTAL at this tiny patch. Thanks!
> 
> lgtm. Thank you for fixing this!

Thanks!

[mtomasz, 2014-09-29 16:19:21]

The CQ bit was checked by mtomasz@chromium.org

[commit-bot: I haz the power, 2014-09-29 16:19:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/595903003/1

[commit-bot: I haz the power, 2014-09-29 16:32:13]

Committed patchset #1 (id:1) as f5ca0ed29deea2886878f5770cf36d4d9b2d273f

[commit-bot: I haz the power, 2014-09-29 16:32:54]

Patchset 1 (id:??) landed as
https://crrev.com/4b84385dafb109ba1cd8e2f78366d3585ddbe059
Cr-Commit-Position: refs/heads/master@{#297188}