GitLab

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}

Review URL: https://codereview.chromium.org/830053004

Cr-Commit-Position: refs/heads/master@{#315231}