Video Player: Fix the condition of 'ended' property

Once the user tries starting the video, the video should not be 'ended'.

BUG=none
TEST=none

Review URL: https://codereview.chromium.org/517963002

Cr-Commit-Position: refs/heads/master@{#292617}

ui/file_manager/video_player/js/cast/cast_video_element.js

@@ -119,7 +119,8 @@ CastVideoElement.prototype = {
     if (!this.castMedia_)
       return true;
 
-   return this.castMedia_.idleReason === chrome.cast.media.IdleReason.FINISHED;
+   return !this.playInProgress &&
+     this.castMedia_.idleReason === chrome.cast.media.IdleReason.FINISHED;
   },
 
   /**