Updated the timestamp XRSession frames provide

The new timestamp is compatible with the one provided to
window.requestAnimationFrame callbacks.

Bug: 864756
Change-Id: Ia68762a150b9d2c4a4760564f8d48aac7c689e94
Reviewed-on: https://chromium-review.googlesource.com/1142227
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Brandon Jones <bajones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576629}
7 files changed