Aframe: Update a-register-element.js

[ks1505]

Description:

Warning fix for aframe Google Chrome,Firefox,IE,Safari (both desktop and mobile)
[Deprecation] document.registerElement is deprecated and will be removed in M73, around March 2019. Please use window.customElements.define instead. See https://www.chromestatus.com/features/4642138092470272
and
https://developer.mozilla.org/en-US/docs/Web/API/Document/registerElement
for more details.
module.exports.registerElement @ a-register-element.js:69
index.js:83 A-Frame Version: 0.8.2 (Date 2018-04-15, Commit #b20527f)
index.js:84 three Version: github:supermedium/three.js#r90fixMTLLoader
index.js:85 WebVR Polyfill Version: ^0.10.5
three.js:20901 THREE.WebGLRenderer 90

Changes proposed:
In a-register-element.js
-Change document.registerElement to
window.customElements.define

[michael-small]

Hello @ks1505,

I noticed this issue too. Here is a previous (closed) issue that makes note of this deprecation.

@dmarcos commented on Dec 17, 2018
It’s ok. We will eventually move to the new custom elements API but we have a polyfill / fallback at the moment on top of Mutation Observers. A-Frame won’t break.

[dmarcos]

This is just a warning and in the case of removal A-Frame will fallback on the polyfill as @michael-small pointed out. There's a WIP PR to move to custom elements V1