Adding WebVR temporary presentation handling on mobile

Temporary code path that invokes fullscreen in lieu of proper VR presentation.
Also includes VRDisplayEvent and related code + casing change to VRPose "timestamp" attribute to match spec.

BUG=389343

Review-Url: https://codereview.chromium.org/2265053004
Cr-Commit-Position: refs/heads/master@{#414165}
12 files changed