Fix IE 11
Fix IE 11

IE 11 seems to be returning Infinity for `performance.now()` on some
systems.  This adds an extra level of validation in `getTimestamp` to
make sure the value is finite.

Fixes #6146

NaderCHASER/cesium,CesiumGS/cesium,oterral/cesium,kaktus40/cesium,CesiumGS/cesium,hodbauer/cesium,progsung/cesium,soceur/cesium,AnalyticalGraphicsInc/cesium,YonatanKra/cesium,CesiumGS/cesium,likangning93/cesium,josh-bernstein/cesium,oterral/cesium,likangning93/cesium,josh-bernstein/cesium,jasonbeverage/cesium,AnalyticalGraphicsInc/cesium,NaderCHASER/cesium,kaktus40/cesium,NaderCHASER/cesium,geoscan/cesium,jasonbeverage/cesium,hodbauer/cesium,CesiumGS/cesium,soceur/cesium,YonatanKra/cesium,likangning93/cesium,YonatanKra/cesium,likangning93/cesium,geoscan/cesium,oterral/cesium,likangning93/cesium,soceur/cesium,YonatanKra/cesium,CesiumGS/cesium,hodbauer/cesium,progsung/cesium