GitLab

Other changes:
- rename anchor and plane pose in mojo to mojo_from_xxxxxx to match convention
- adjust samples to account for the possibility that anchors and planes
  can have their poses unknown
- send null poses from device if tracking is paused
- ARCore device now does not treat planes and anchors whose tracking is
  paused as removed
- add DVLOGs all around to help with issue investigation
- add test page for experiments with local space stability & drift

Note: the test mentioned above is not exposed via proposals/index.html.
Change-Id: I786e86d69b69badbace57ce62585e3d5fd1ae756
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2130778Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Commit-Queue: Piotr Bialecki <bialpio@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757560}