Add UMA metrics for VideoCodec.MP4 and VideoCodec.WebM

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 d476b29069e83525241378e66c58b9d231e3693f..4fdd4c7bae981764c5b55996b973a8bed921dee3 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -27825,6 +27825,22 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Media.MSE.VideoCodec.MP4" enum="MSECodec">
+  <owner>media-dev@chromium.org</owner>
+  <summary>
+    Video codec used in Media Source Extensions playback if the media container
+    is MP4. Set when AddId() is called during playback.
+  </summary>
+</histogram>
+
+<histogram name="Media.MSE.VideoCodec.WebM" enum="MSECodec">
+  <owner>media-dev@chromium.org</owner>
+  <summary>
+    Video codec used in Media Source Extensions playback if the media container
+    is WebM. Set when AddId() is called during playback.
+  </summary>
+</histogram>
+
 <histogram name="Media.Netflix.AudioBitrate" units="kbps">
   <obsolete>
     Deprecated 04/2016 as doesn't have data nor owner.
@@ -28191,6 +28207,20 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Media.SRC.VideoCodec.MP4" enum="VideoCodec">
+  <owner>media-dev@chromium.org</owner>
+  <summary>
+    Video codec used in HTML5 media if the media container is MP4.

[xhwang, 2017/05/04 18:34:53]

nit: Mention this is only for SRC playback.

[kqyang, 2017/05/04 19:01:41]

Done.

+  </summary>
+</histogram>
+
+<histogram name="Media.SRC.VideoCodec.WebM" enum="VideoCodec">
+  <owner>media-dev@chromium.org</owner>
+  <summary>
+    Video codec used in HTML5 media if the media container is WebM.

[xhwang, 2017/05/04 18:34:53]

ditto

[wolenetz, 2017/05/04 18:46:00]

nit here and in the .SRC., above: I think the forthcoming Media.ALL.* (see bug
716183) should have this text, and the .SRC. metrics added here should be more
specific.

xhwang@, WDYT about "Video codec used in plain src= (not MSE) HTML5 media if the
media container is {WebM,MP4}" ?

We'll also (eventually) want to be even more clear about which metrics might
count things in multiple places (ALL and SRC/MSE and EME, and maybe EME and
SRC/MSE) -- this further clarification can and should happen as part of fixing
crbug 716183 IMHO.

[kqyang, 2017/05/04 19:01:41]

Done.

+  </summary>
+</histogram>
+
 <histogram base="true" name="Media.Timeline.DragGestureDuration" units="ms">
 <!-- Name completed by histogram_suffixes name="MediaTimelineWidths" -->