cros: Add a11y to enable debugging screen.

cros: Add a11y to enable debugging screen.

- Fix defaultControl to return ok button for error screen;
- Add aria-live and aria-atomic to content div so that it is read out
  when the screen state is changed and all contents are updated;

BUG=413766
TEST=Test enable-debugging flow with ChromeVox and verify that title and message gets read on each step.

Committed: https://crrev.com/a63b2ff95caa18852ef3448db213ee730cbd6bab
Cr-Commit-Position: refs/heads/master@{#311542}

[xiyuan, 2015-01-13 21:20:26]

xiyuan@chromium.org changed reviewers:
+ dtseng@chromium.org

[xiyuan, 2015-01-13 21:20:27]

David, could you help with the review? Thanks.

[David Tseng, 2015-01-14 19:16:04]

lgtm

nit: please add a TEST= line (e.g. tested with ChromeVox; verified the enabled
debugging message gets read after ...).

[xiyuan, 2015-01-14 19:20:26]

On 2015/01/14 19:16:04, David Tseng wrote:
> lgtm
> 
> nit: please add a TEST= line (e.g. tested with ChromeVox; verified the enabled
> debugging message gets read after ...).

TEST line added.

[xiyuan, 2015-01-14 19:20:37]

The CQ bit was checked by xiyuan@chromium.org

[commit-bot: I haz the power, 2015-01-14 19:21:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/788953005/1

[commit-bot: I haz the power, 2015-01-14 20:52:24]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-14 20:53:25]

Patchset 1 (id:??) landed as
https://crrev.com/a63b2ff95caa18852ef3448db213ee730cbd6bab
Cr-Commit-Position: refs/heads/master@{#311542}