UserAgent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/70.0.3538.67 Safari/537.36

Steps to reproduce the problem:
This is also reproducible in [this googlecodelabs demo](https://googlecodelabs.github.io/ar-with-webxr/final/) with the following codes to be run in console sequentially.

```
// 0. Initialize vars
navigator.getUserMedia = (navigator.getUserMedia || navigator.webkitGetUserMedia || navigator.mozGetUserMedia || navigator.msGetUserMedia);
var gumOption = {
  video: {
    facingMode: "environment",
    height: 480,
    width: 640,
  },
  audio:false
};

// 1. Initialize a function to end XR session
async function endXr () {
  // Cancel the current RAF
  app.session.cancelAnimationFrame(app.onXRFrame);

  // End the XR session
  await app.session.end();

  // Unbind the framebuffer to baseLayer's framebuffer
  app.gl.bindFramebuffer(app.gl.FRAMEBUFFER, null);

  // Remove the previously created canvas in 'onEnterAR()'
  var canvas = document.getElementsByTagName('canvas')[0];
  canvas.parentNode.removeChild(canvas);

  // Remove window's click bound event
  window.removeEventListener('click', app.onClick);

  // Revert frame buffer back to how it previously was.
  THREE.Object3D.prototype.onBeforeRender = () => {}
}

// 2. Stop XR session
endXr();

// 3. Run GUM
navigator.getUserMedia(gumOption, function(stream) {
    window.stream = stream;
    console.log(":: Got stream !!", stream);
  }, function(error) {
    console.error(":: Got error instead 8(", error);
  }
);

// 4. Stop GUM
stream.getTracks().forEach(stream => stream.stop());

// 5. To restart the XR
app.onEnterAR();
```

In the code above, I have provided my approach to end the XRSession (**1st**) and to restart it back up (**5th**). And if you follow those steps sequentially in [that googlecodelabs demo](https://googlecodelabs.github.io/ar-with-webxr/final/), you may notice that after the **5th** step, WebXR freezes and becomes unusable.

Now, this issue does not happen if I just end the XRSession and restart it back. But only happens if I include `getUserMedia` operation which is the **3rd** and **4th**.

Another thing is that if `gumOption.video.facingMode` is not provided, `getUserMedia` fails in the **3rd** step.

What is the expected behavior?
Re-entering WebXR should work

What went wrong?
WebXR freezes and becomes unusable after getUserMedia being used in the page.

Did this work before? No 

Does this work in other browsers? No
 WebXR freezes and becomes unusable after getUserMedia being used in the page.

Chrome version: 71.0.3574.0  Channel: dev
OS Version: 16.04
Flash Version: