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

Review URL: https://codereview.chromium.org/272803003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@269429 0039d316-1c4b-4281-b951-d872f2087c98

• content/browser/renderer_host/legacy_render_widget_host_win.cc[diff]