Make interstitial pages accessible.

WebContentsImpl maintains both a real page, and also optionally
an interstitial page that's shown instead of the main page
if the site isn't safe. The interstitial wasn't accessible,
we were still exposing the main page via accessibility APIs
even if an interstitial was showing visually.

This change modifies WebContentsImpl so that if an interstitial
is visible, that interstitial is exposed instead of the main page,
matching what's shown visually. When an interstitial
is attached or detached, ensure that focus events are fired,
and ensure that the page hidden by the interstitial can't fire
any accessibility events.

Adds a browser test to ensure this works.

Bug: 730910
Change-Id: I471ca92c0563fb803ada182edd96675d0a9a0cad
Reviewed-on: https://chromium-review.googlesource.com/830619
Reviewed-by: Jochen Eisinger <[email protected]>
Reviewed-by: Nektarios Paisios <[email protected]>
Reviewed-by: Emily Stark <[email protected]>
Commit-Queue: Dominic Mazzoni <[email protected]>
Cr-Commit-Position: refs/heads/master@{#525545}
8 files changed