Polyfill performance.now on global object

[brianchirls]

If cannon.js is included in a script that runs in strict mode, the polyfill of performance will throw an error because it's undeclared. You can see this by adding "use strict" to World.js or the built script and running the web workers example in Mobile Safari (iOS only supports performance in the main thread, not in a worker) or running the unit test (node doesn't support performance at all).

In browsers, the polyfill needs to be set on self in either worker or the main thread. In node, it needs to be set on global.