Add cancel() method for hit test sources

This CL adds cancel() method to hit test sources. Details can be found
here:
https://github.com/immersive-web/hit-test/blob/master/hit-testing-explainer.md#unsubscribing-from-hit-test

Change-Id: Ia988497ebb26b4c5b76f71de323398593b883b5f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1937140
Commit-Queue: Piotr Bialecki <bialpio@chromium.org>
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719403}
13 files changed