Adds histograms for casting feature of Video Player

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

Index: ui/file_manager/video_player/js/cast/cast_video_element.js
diff --git a/ui/file_manager/video_player/js/cast/cast_video_element.js b/ui/file_manager/video_player/js/cast/cast_video_element.js
index a2ffb3904438a2246531ab8e510810771b2bd78a..c3559fc05e699078101851847cd5734694a17ca7 100644
--- a/ui/file_manager/video_player/js/cast/cast_video_element.js
+++ b/ui/file_manager/video_player/js/cast/cast_video_element.js
@@ -227,6 +227,8 @@ CastVideoElement.prototype = {
       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;
       }