Fix mounting read-only volumes in Files app.

Fix mounting read-only volumes in Files app.

The new logic was trying to mount all volumes in R/W mode, which was failing
for read-only volumes.

TEST=Tested manually with read only volumes.
BUG=None

Committed: https://crrev.com/1e91e5fc31e45c219ac57366b058e4890bb044bd
Cr-Commit-Position: refs/heads/master@{#321938}

[mtomasz, 2015-03-24 02:08:44]

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

[mtomasz, 2015-03-24 05:02:27]

@hirono: PTAL. Thanks.

[hirono, 2015-03-24 05:26:43]

lgtm, sorry for missing it. Thanks!

[mtomasz, 2015-03-24 05:26:58]

On 2015/03/24 05:26:43, hirono wrote:
> lgtm, sorry for missing it. Thanks!

thanks!

[mtomasz, 2015-03-24 05:27:01]

The CQ bit was checked by mtomasz@chromium.org

[commit-bot: I haz the power, 2015-03-24 05:27:15]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1024553005/1

[commit-bot: I haz the power, 2015-03-24 05:46:35]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-24 05:47:23]

Patchset 1 (id:??) landed as
https://crrev.com/1e91e5fc31e45c219ac57366b058e4890bb044bd
Cr-Commit-Position: refs/heads/master@{#321938}