Added metrics for background video playback.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 5b8d8d55d0932452d28a99350da435a2388f3f5e..a8e35ca76df067800c978fc218d0672813618f14 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -22856,6 +22856,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Media.Session.BackgroundTime" units="ms">
+  <owner>avayvod@chromium.org</owner>
+  <owner>mlamouri@chromium.org</owner>

[mlamouri (slow - plz ping), 2016/08/08 13:19:31]

Maybe add zqzhang@?

[whywhat, 2016/08/11 21:51:04]

Done.

+  <summary>
+    Time between the moment a backgrounded video is resumed by the user and

[Ilya Sherman, 2016/08/08 22:09:10]

Hmm, does the user explicitly need to resume the video whenever it is
backgrounded?  If not, and the video was backgrounded, continued to play, and
then was foregrounded again, will that duration be recorded in this histogram?

[whywhat, 2016/08/11 21:51:04]

Yes. Clarified that in the new description.

If not, and the video was backgrounded, continued to play, and then was
foregrounded again, will that duration be recorded in this histogram?

No.

+    until it's either paused, stopped or brought back to the foreground.
+  </summary>
+</histogram>
+
 <histogram name="Media.Session.Pause" enum="MediaSessionActionSource">
   <owner>avayvod@chromium.org</owner>
   <owner>mlamouri@chromium.org</owner>