fix(progress-circle): correct elapsed time calculation (#927)

Fixes the progress circle animation not working, because the calculations weren't correct. When 14c1765 was introduced, one of the values uses the time since epoch, whereas the other one is the time since the page was opened.

Fixes #926.

Author: crisbeto

src/components/progress-circle/progress-circle.ts

@@ -158,9 +158,8 @@ export class MdProgressCircle implements OnDestroy {
     if (animateTo === animateFrom) {
       this.currentPath = getSvgArc(animateTo, rotation);
     } else {
-      let animation = (currentTime: number) => {
-        let elapsedTime = Math.max(
-          0, Math.min((currentTime || Date.now()) - startTime, duration));
+      let animation = () => {
+        let elapsedTime = Math.max(0, Math.min(Date.now() - startTime, duration));
 
         this.currentPath = getSvgArc(
           ease(elapsedTime, animateFrom, changeInValue, duration),