GitLab

Latest spec: https://immersive-web.github.io/anchors/

- remove last changed time from anchor object
- anchor space is not nullable on anchor object now
- anchor objects are not created in zombie state anymore - when anchor
  creation promise resolves, it should receive a fully built object
- XRAnchor got simplified due to the above
- XRFrameProvider needs to make sure that JS promises can run between
  presentation frame update and rAF callbacks so that the app has a chance
  to see newly created anchors before they are delivered to it via
  XRFrame.trackedAnchors

Changes: 
Change-Id: If582552907274d472097f7d7b119bb7ee216736e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2085701
Commit-Queue: Piotr Bialecki <bialpio@chromium.org>
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747901}