New API to test table headers at specified index.

Adds accessibility API to test table headers.

API is used to test table headers at particular index.

BUG=441706
Committed: https://crrev.com/aaddbff8927212373c832842d5b1d5432ab7b1d0
Cr-Commit-Position: refs/heads/master@{#309758}

[k.czech, 2014-12-16 19:18:45]

k.czech@samsung.com changed reviewers:
+ dmazzoni@chromium.org

[dmazzoni, 2014-12-17 01:01:26]

The CQ bit was checked by dmazzoni@chromium.org

[dmazzoni, 2014-12-17 01:01:26]

lgtm

[commit-bot: I haz the power, 2014-12-17 01:02:02]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/796373002/1

[commit-bot: I haz the power, 2014-12-17 01:17:04]

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

[commit-bot: I haz the power, 2014-12-17 01:17:04]

Try jobs failed on following builders:
  android_dbg_tests_recipe on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/android_dbg_tes...)
  linux_chromium_gn_dbg on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)
  mac_chromium_compile_dbg_ng on tryserver.chromium.mac
(http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_comp...)

[k.czech, 2014-12-19 13:11:25]

On 2014/12/17 01:17:04, I haz the power (commit-bot) wrote:
> Try jobs failed on following builders:
>   android_dbg_tests_recipe on tryserver.chromium.linux
>
(http://build.chromium.org/p/tryserver.chromium.linux/builders/android_dbg_tes...)
>   linux_chromium_gn_dbg on tryserver.chromium.linux
>
(http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)
>   mac_chromium_compile_dbg_ng on tryserver.chromium.mac
>
(http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_comp...)

This one depends on the https://codereview.chromium.org/797223002/ change. It
should go in after.

[k.czech, 2014-12-30 11:12:57]

The CQ bit was checked by k.czech@samsung.com

[commit-bot: I haz the power, 2014-12-30 11:13:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/796373002/20001

[commit-bot: I haz the power, 2014-12-30 11:55:39]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2014-12-30 11:56:36]

Patchset 2 (id:??) landed as
https://crrev.com/aaddbff8927212373c832842d5b1d5432ab7b1d0
Cr-Commit-Position: refs/heads/master@{#309758}