Add back the ALERT trick to detect screen readers.

Add back the ALERT trick to detect screen readers.

This fires an EVENT_OBJECT_ALERT notification on the legacy hwnd
with a custom id, and if we get a response we enable full web
accessibility.

This code used to be in render_widget_host_view_win. The replacement
code was not working reliably for unknown reasons, and other accessibility
clients were relying on the previous trick - see bugs.

BUG=319523, 342319
NOTRY=true
R=ananta@chromium.org, creis@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=269429

[ananta, 2014-05-08 22:22:33]

lgtm

[dmazzoni, 2014-05-08 22:26:22]

+creis for owners review

[Charlie Reis, 2014-05-08 22:29:39]

LGTM.  Is there a way to test this, or is that tricky because of the external
screen reader?

[dmazzoni, 2014-05-08 22:34:10]

Thanks.

I tested it manually by adding some logging to
make sure it was triggering with NVDA or JAWS is
running and not triggering when they're not.

I don't see anything worth more unit testing here.
We have good test coverage for what happens
when you enable accessibility via calling
BrowserAccessibilityState.

[dmazzoni, 2014-05-08 22:37:43]

The CQ bit was checked by dmazzoni@chromium.org

[commit-bot: I haz the power, 2014-05-08 22:39:22]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/dmazzoni@chromium.org/272803003/1

[dmazzoni, 2014-05-09 13:36:23]

The CQ bit was unchecked by dmazzoni@chromium.org

[dmazzoni, 2014-05-09 13:36:46]

The CQ bit was checked by dmazzoni@chromium.org

[commit-bot: I haz the power, 2014-05-09 13:42:15]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/dmazzoni@chromium.org/272803003/1

[dmazzoni, 2014-05-09 21:39:10]

The CQ bit was unchecked by dmazzoni@chromium.org

[dmazzoni, 2014-05-09 22:02:42]

Committed patchset #2 manually as r269429 (presubmit successful).