Add VisiblePosition validity checks to VisibleUnits.cpp

Add VisiblePosition validity checks to VisibleUnits.cpp

BUG=648949
Committed: https://crrev.com/1985713ab7e640a5d5de02a579e10aed85eae34a
Cr-Commit-Position: refs/heads/master@{#421064}

[Xiaocheng, 2016-09-26 03:59:56]

The CQ bit was checked by xiaochengh@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-09-26 04:00:08]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[Xiaocheng, 2016-09-26 04:49:43]

xiaochengh@chromium.org changed reviewers:
+ yosin@chromium.org

[Xiaocheng, 2016-09-26 04:49:44]

PTAL.

[yosin_UTC9, 2016-09-26 05:22:54]

lgtm

[commit-bot: I haz the power, 2016-09-26 06:42:54]

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

[commit-bot: I haz the power, 2016-09-26 06:42:55]

Dry run: Try jobs failed on following builders:
  linux_android_rel_ng on master.tryserver.chromium.android (JOB_FAILED,
https://build.chromium.org/p/tryserver.chromium.android/builders/linux_androi...)

[Xiaocheng, 2016-09-26 23:39:28]

The CQ bit was checked by xiaochengh@chromium.org

[commit-bot: I haz the power, 2016-09-26 23:39:58]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-09-27 01:08:25]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2016-09-27 01:10:16]

Description was changed from

==========
Add VisiblePosition validity checks to VisibleUnits.cpp

BUG=648949
==========

to

==========
Add VisiblePosition validity checks to VisibleUnits.cpp

BUG=648949

Committed: https://crrev.com/1985713ab7e640a5d5de02a579e10aed85eae34a
Cr-Commit-Position: refs/heads/master@{#421064}
==========

[commit-bot: I haz the power, 2016-09-27 01:10:17]

Patchset 1 (id:??) landed as
https://crrev.com/1985713ab7e640a5d5de02a579e10aed85eae34a
Cr-Commit-Position: refs/heads/master@{#421064}