Update navigator.xr.requestSession to take a mode directly

Updates Chrome's WebXR implementation to match the latest spec changes.
Also means the removal of a file and slight reduction of code for most
of the tests and samples!

Bug: 953450
Change-Id: Ia0cbe48668909edbc9a2ff388a03e2a880c79bad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1570217
Commit-Queue: Brandon Jones <bajones@chromium.org>
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#651936}
81 files changed