GitLab

The mapping is defined by the spec at:
https://immersive-web.github.io/webxr/#xr-standard-gamepad-mapping
Now, only a primary trigger is required for a controller to claim
xr-standard mapping, and the order of some of the buttons has changed.

Centralize the logic of how to order buttons + input axes, as well as
where/when to add placeholder values in the XRStandardGamepadBuilder
class.

Bug: 979246
Change-Id: Iefb116ea603c0a7237575d189686fd2daee9de90
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1728345
Commit-Queue: Jacob DeWitt <jacde@chromium.org>
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#684446}