Window performance fix

This piece of code here:

https://github.com/mrdoob/three.js/blob/master/utils/npm/header.js#L16

Makes the assumption that if ```window.performance``` or ```window.performance.now``` doesn't exist that ```process.hrtime()``` does exist.  This is not necessarily the case.  Some browsers just don't yet support ```windows.performance``` I understand or someone may screw up ```windows.performance``` so that it no longer exists.

My suggested fix is to use ```new Date().getTime();``` when ```process.httime()``` doesn't exist.