Bug 218305

To reproduce: 
Create over 16 WebGL contexts and call loseContext on each.
On the 17th call two errors will be thrown: 
> ‘There are too many active WebGL contexts on this page, the oldest context will be lost.’
> ‘WebGL: INVALID_OPERATION: loseContext: context already lost’
It's expected that these contexts would not remain in memory.

Code snippet:
for (let i = 0; i < 17; i++) {
  const canvas = document.createElement('canvas');
  const gl = canvas.getContext('webgl');  
  const extension = gl.getExtension('WEBGL_lose_context')
  extension.loseContext();
}

Example CodePen of dynamically losing & restoring contexts.
https://codepen.io/philipbell/pen/RwRQGmo