Fix accessibility tests on Mac 10.10

Fix accessibility tests on Mac 10.10

The role description for "check box" is inconsistent between versions, so
use our own string rather than the system string.

AccessibilityInputDateTimeLocal is too OS-version-dependent, just disable it
on Mac for now, as it has some coverage on other platforms.

BUG=513883
Committed: https://crrev.com/bd1d6438cb77461e99edb3bae1623e6dbbc56c27
Cr-Commit-Position: refs/heads/master@{#342466}

[dmazzoni, 2015-08-07 19:49:52]

dmazzoni@chromium.org changed reviewers:
+ avi@chromium.org, erikchen@chromium.org

[Avi (use Gerrit), 2015-08-07 20:18:00]

lgtm

ok

[dmazzoni, 2015-08-07 20:54:44]

The CQ bit was checked by dmazzoni@chromium.org

[commit-bot: I haz the power, 2015-08-07 20:55:16]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1278203002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1278203002/1

[commit-bot: I haz the power, 2015-08-07 22:45:21]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-07 22:46:07]

Patchset 1 (id:??) landed as
https://crrev.com/bd1d6438cb77461e99edb3bae1623e6dbbc56c27
Cr-Commit-Position: refs/heads/master@{#342466}