Adds histograms for casting feature of Video Player

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

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * Interval for updating media info (in ms).
  * @type {number}
  * @const
  */
 var MEDIA_UPDATE_INTERVAL = 250;
@@ skipping 209 matching lines @@
   play: function(opt_seeking) {
     if (this.playInProgress_)
       return;
 
     var play = function() {
       // If the casted media is already playing and a pause request is not in
       // progress, we can skip this play request.
       if (this.castMedia_.playerState ===
               chrome.cast.media.PlayerState.PLAYING &&
           !this.pauseInProgress_) {
+        metrics.recordCastedVideoLength(this.currentMediaDuration_);

[fukino, 2014/12/10 08:01:47]

It seems that the code here is for avoiding sending redundant play request. I
couldn't understand why this point can record the length of each casted video
correctly...

[yoshiki, 2014/12/10 09:33:19]

Sorry, I added it the wrong place. Fixed.

[fukino, 2014/12/10 10:16:42]

IIRC, when a user repeat PAUSE->PLAY->PAUSE->PLAY, the duration will be recorded
multiple times.
How about inside onMediaDiscovered_()?

[fukino, 2014/12/10 12:18:31]

It looks OK to record here, given that covering cases in which multiple media
are casted will be complex.

+
         this.playInProgress_ = false;
         return;
       }
 
       var playRequest = new chrome.cast.media.PlayRequest();
       playRequest.customData = {seeking: !!opt_seeking};
 
       this.castMedia_.play(
           playRequest,
           function() {
@@ skipping 250 matching lines @@
       this.dispatchEvent(new Event('durationchange'));
     }
 
     // Media is being unloaded.
     if (!alive) {
       this.unloadMedia_();
       return;
     }
   },
 };