Update WebVR interface to match the 1.1 spec

This update doesn't change the VR API backends at all, just transforms
the data that they're currently providing to match the WebVR 1.1 spec.
A future CL will plumb the changes all the way back to /device/vr.

BUG=389343

Review-Url: https://codereview.chromium.org/2331553002
Cr-Commit-Position: refs/heads/master@{#418153}
14 files changed