Notify user of HTML validation error messages

Make HTML native validation of form controls accessible

A simple example is @required on a form control. Blink provides a
helpful message box if the user attempts to submit a form with a
required field left empty. The control is also focused.

This CL produces an accessible alert for the notification when it appears.

Bug: 897039
Change-Id: Ib78ce09821d794c8b35ca151c1f6a153adff9414
Reviewed-on: https://chromium-review.googlesource.com/c/1403980
Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625551}
26 files changed