Add runtime flag to gate WebXR Anchors feature

Bug: 992025
Change-Id: I8f0a9386078f61aa25d48219eb42bd336011bb13
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1744519
Reviewed-by: Kentaro Hara <[email protected]>
Reviewed-by: Alex Moshchuk <[email protected]>
Commit-Queue: Piotr Bialecki <[email protected]>
Cr-Commit-Position: refs/heads/master@{#685627}
11 files changed