Reland Various changes required to support ChromeVox Next to read Views and Windows.

Original change
https://codereview.chromium.org/743273002/

Modifications
Removes unneeded AlertWindow class.

TEST=chromevox_tests
[email protected]

Review URL: https://codereview.chromium.org/766473003

Cr-Commit-Position: refs/heads/master@{#306891}
22 files changed