fix(ngAnimate): do not use event.timeStamp anymore for time tracking

Due to recent changes in Chrome, Firefox and Webkit use of the
event.timeStamp value will lead to unpredictable behaviour due to
precision changes. Therefore it's best to stick entirely to use
`Date.now()` when it comes to confirming the end of transition-
ending values. See angular#13494 for more info.

Applies to 1.2, 1.3, 1.4 and 1.5.

Closes angular#13494
Closes angular#13495

Author: matsko

src/ngAnimate/animate.js

@@ -1956,15 +1956,15 @@ angular.module('ngAnimate', ['ng'])
         function onAnimationProgress(event) {
           event.stopPropagation();
           var ev = event.originalEvent || event;
-          var timeStamp = ev.$manualTimeStamp || ev.timeStamp || Date.now();
+          var timeStamp = ev.$manualTimeStamp || Date.now();
 
           /* Firefox (or possibly just Gecko) likes to not round values up
            * when a ms measurement is used for the animation */
           var elapsedTime = parseFloat(ev.elapsedTime.toFixed(ELAPSED_TIME_MAX_DECIMAL_PLACES));
 
           /* $manualTimeStamp is a mocked timeStamp value which is set
            * within browserTrigger(). This is only here so that tests can
-           * mock animations properly. Real events fallback to event.timeStamp,
+           * mock animations properly. Real events fallback to Date.now(),
            * or, if they don't, then a timeStamp is automatically created for them.
            * We're checking to see if the timeStamp surpasses the expected delay,
            * but we're using elapsedTime instead of the timeStamp on the 2nd