Implement hit-test feature descriptor & policy, add WPTs

- hit test subscription should fail when the session has ended
- implement feature descriptor and feature policy for hit test
- add WPTs to check whether subscription succeeds/fails appropriately

Changes: 
Change-Id: Ib7735ca3c1015a4e3742c71d6d144e24a79c935e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2019608
Commit-Queue: Piotr Bialecki <bialpio@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735727}
14 files changed