Add Accessibility Object Model to content features, and add a browser test.

More tests at the content layer will follow, this just sets up the
infrastructure for future tests.

BUG=680345
TEST=Run Chrome with --enable-features=AccessibilityObjectModel, print document.body.accessibleNode in Inspector

Review-Url: https://codereview.chromium.org/2787843003
Cr-Commit-Position: refs/heads/master@{#461978}
13 files changed