Add conversion rule for aria-grabbed attributes.

This patch adds conversion rule for aria-grabbed attribute and
exposes it only when it's defined.

The related patch is
https://codereview.chromium.org/736943002/

BUG=98817

Review URL: https://codereview.chromium.org/736023002

Cr-Commit-Position: refs/heads/master@{#313349}
13 files changed