perf of native WebVR API tracking degraded on Chrome for Android

[cvan]

reproduced in Chrome 65 on an Android Pixel 2 device:

0. ensure that the WebVR flag is Enabled from chrome://flags#enable-webvr
1. using Chrome on Android (current release: 65), navigate to the demo scene at https://mozilla.github.io/unity-webvr-export/ (or your localhost server)
2. click the VR-icon button to enter VR mode.
3. move your head around a ton, up and forth, left and right.
4. notice the tracking is really lagged. the perf was noticeably better.

likely, this appears to be mostly caused the upgrade of the WebVR Polyfill in v1.1.0.

the actionable steps here are to test other WebVR content in Chrome for Android, and see if this is a platform regression or a regression in this project/repo. if it's the former, we should file Chromium bugs. if it's the latter, we should fix the issue in this project.

also, see this relevant issue for Polyfill issues with the new Sensor API: #254