Implemented full support for NSAccessibilityLinkedUIElementsAttribute.

1. In-page links now work, e.g. in the table of contents of a Wikipedia article.
2. Both native and ARIA radio buttons behave more consistently.
BUG=37721
R=dmazzoni@chromium.org, ellyjones@chromium.org
TESTED=layout test, browser tests, manually with Voiceover

Review-Url: https://codereview.chromium.org/2734923005
Cr-Commit-Position: refs/heads/master@{#457695}
30 files changed