[Video] Collect average keyframe distance for video

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 487b1ab170bdf8d93c9dc5d8790e6f9a72a2755e..a4e855db7a14b835e6412b0b45ae474060a09a01 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -25375,6 +25375,17 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Media.Video.KeyFrameDistanceAverage" units="ms">
+  <owner>avayvod@chromium.org</owner>
+  <owner>dalecurtis@chromium.org</owner>
+  <summary>
+    Records the moving average of time distance between consequent keyframes in
+    a video based on the last N frames (see N in decoder_stream_traits.cc). The
+    new average is calculated for each video key frame decoded and is recorded
+    every time the average increases from the previous one.
+  </summary>
+</histogram>
+
 <histogram name="Media.VideoCapture.AspectRatio" units="%">
   <owner>mcasas@chromium.org</owner>
   <summary>