Revert of Suppress accessibility events when user is navigating away.

Revert of Suppress accessibility events when user is navigating away. (patchset #6 id:100001 of https://codereview.chromium.org/830053004/)

Reason for revert:
NavigationAccessibilityTest.TestNavigateToNewUrl is failing on Win7 Tests (dbg)(1).

Sample failure:
http://build.chromium.org/p/chromium.win/builders/Win7%20Tests%20%28dbg%29%281%29/builds/35087

Original issue's description:
> Suppress accessibility events when user is navigating away.
>
> When the user navigates to a new page, stop sending accessibility
> events on the old page.
>
> BUG=421116, 450409
>
> Committed: https://crrev.com/6ce40a1e561892849c1f6ac070dda140f6cc0115
> Cr-Commit-Position: refs/heads/master@{#314812}

TBR=nasko@chromium.org,dmazzoni@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=421116, 450409
Committed: https://crrev.com/7fcb729df6369250da0eccb113a64073e272de39
Cr-Commit-Position: refs/heads/master@{#314884}

[kelvinp, 2015-02-05 21:36:05]

Created Revert of Suppress accessibility events when user is navigating away.

[commit-bot: I haz the power, 2015-02-05 21:36:39]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/892513004/1

[commit-bot: I haz the power, 2015-02-05 21:37:54]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-05 21:40:16]

Patchset 1 (id:??) landed as
https://crrev.com/7fcb729df6369250da0eccb113a64073e272de39
Cr-Commit-Position: refs/heads/master@{#314884}