Add test infrastructure for the file-picker modal test

Add test infrastructure for the file-picker modal test

This CL allows the unit test to directly open and close a Gtk
file-picker, which is used to run the file-picker modal test as follows:
https://codereview.chromium.org/1233913009/

BUG=None

Committed: https://crrev.com/3d1b89d8b07530fa86e102bb4f64b61527dceee4
Cr-Commit-Position: refs/heads/master@{#360716}

[joone, 2015-11-20 00:06:13]

joone.hur@intel.com changed reviewers:
+ sky@chromium.org

[joone, 2015-11-20 00:06:13]

This is the follow-up on this comment:
https://codereview.chromium.org/1233913009/#msg76

sky@ could you review this CL?

[sky, 2015-11-20 01:03:00]

LGTM

[joone, 2015-11-20 01:03:46]

The CQ bit was checked by joone.hur@intel.com

[commit-bot: I haz the power, 2015-11-20 01:07:02]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1462903004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1462903004/1

[commit-bot: I haz the power, 2015-11-20 01:22:40]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-11-20 01:22:41]

Try jobs failed on following builders:
  ios_dbg_simulator_ninja on tryserver.chromium.mac (JOB_TIMED_OUT, no build
URL)
  ios_rel_device_ninja on tryserver.chromium.mac (JOB_TIMED_OUT, no build URL)

[joone, 2015-11-20 01:43:29]

The CQ bit was checked by joone.hur@intel.com

[commit-bot: I haz the power, 2015-11-20 01:46:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1462903004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1462903004/1

[commit-bot: I haz the power, 2015-11-20 02:01:44]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-20 02:03:28]

Patchset 1 (id:??) landed as
https://crrev.com/3d1b89d8b07530fa86e102bb4f64b61527dceee4
Cr-Commit-Position: refs/heads/master@{#360716}