Use performance.now() for the frames instead (#859)

I'm not sure how `Date.now()` sneaked in there. Dates should only ever
be used if the calendar is important, which is definitely not the case
for managing the frame rate. Any NTP sync should haved messed with the
frames before, at least to some slight degree.

src/util/AutoRefresh.tsx

@@ -31,7 +31,7 @@ export default class AutoRefresh extends React.Component<Props> {
     }
 
     private startAnimation() {
-        this.previousTime = Date.now();
+        this.previousTime = performance.now();
         this.animate();
     }
 
@@ -40,7 +40,7 @@ export default class AutoRefresh extends React.Component<Props> {
 
         assert(this.previousTime !== undefined, "Previous time must be defined");
 
-        const currentTime = Date.now();
+        const currentTime = performance.now();
         const elapsed = currentTime - this.previousTime;
 
         if (elapsed > this.fpsInterval) {