Fix chromium bug (frozen chaperone)

Summary:
## Motivation (required)

Same issue as [this](toji/chrome-webvr-issues#64 (comment)). Basically chaperone gets frozen and nothing else gets rendered. Also got this warning message in the console: `GL ERROR :GL_INVALID_OPERATION : glFramebufferTexture2D`.

The same issue was solved by adding `alpha: true` when constructing the `WebGLRenderer`.

## Test Plan (required)

I've only got a Vive so needs to be tested on other non-openVR platforms.
Closes #161

Reviewed By: mikearmstrong001

Differential Revision: D4963586

Pulled By: andrewimm

fbshipit-source-id: f62864b

OVRUI/src/Player/Player.js

@@ -184,8 +184,10 @@ export default class Player {
 
     // Set up the Three.js basics: renderer, camera, controls
     const antialias = options.hasOwnProperty('antialias') ? options.antialias : true;
+    const alpha = options.hasOwnProperty('canvasAlpha') ? options.canvasAlpha : true;
     const renderer = new THREE.WebGLRenderer({
       antialias: antialias,
+      alpha: alpha,
     });
     renderer.setPixelRatio(pixelRatio);
     renderer.setSize(width, height);