VideoSynchronizer flicker outside of video duration

Reported on the forum: https://groups.google.com/forum/?hl=en#!topic/cesium-dev/guKaP3J6n4k

If `videoElement.loop = false;` and the video is synchronized with the clock using a `VideoSynchronizer` and the time is outside of the range, the video flickers between the start and end of the video.

To reproduce:
- http://cesiumjs.org/Cesium/Apps/Sandcastle/index.html?src=Video.html&label=Showcases
- add `videoElement.loop = false;` to line 4 and run the example
- click `Toggle clock synchronization` and `Toggle Video Overlay`
- In the video overlay, you will see the slider flicker back and forth between the `0` and `32` second mark

Start with the Video.html Sandcastle example and reproduce the issue after setting videoElement.loop = false, then enable clock synchronization and the video overlay. Read the VideoSynchronizer behavior around times outside the video duration. Done means the overlay no longer flickers between 0 and 32 seconds.

Written by the indexing model from the issue text.