media: Record counts of detected src= audio, video, and text tracks

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 a6777275a88188e9ef9bd0de4e4a23ecca919ed9..bb256604099942615f67e85501ffda0cdb57d44e 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -19762,7 +19762,11 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 
 <histogram name="Media.DetectedAudioCodecHash" enum="FFmpegCodecHashes">
   <owner>jrummell@chromium.org</owner>
-  <summary>Audio codec used in HTML5 media.</summary>
+  <summary>
+    Audio codec used in HTML5 media. Reporting corrected in M51 to include all
+    detected audio codecs, not just those up to and including the first one
+    supported.
+  </summary>
 </histogram>
 
 <histogram name="Media.DetectedContainer" enum="MediaContainers">
@@ -19774,6 +19778,30 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Media.DetectedTrackCount.Audio">
+  <owner>wolenetz@chromium.org</owner>
+  <summary>
+    Number of detected audio tracks in HTML5 media. Not all may be usable by the
+    player.
+  </summary>
+</histogram>
+
+<histogram name="Media.DetectedTrackCount.Text">
+  <owner>wolenetz@chromium.org</owner>
+  <summary>
+    Number of detected text tracks in HTML5 media. Not all may be usable by the
+    player.
+  </summary>
+</histogram>
+
+<histogram name="Media.DetectedTrackCount.Video">
+  <owner>wolenetz@chromium.org</owner>
+  <summary>
+    Number of detected video tracks in HTML5 media. Not all may be usable by the
+    player.
+  </summary>
+</histogram>
+
 <histogram name="Media.DetectedVideoCodec" enum="FFmpegCodecs">
   <obsolete>
     Deprecated Sep 15 2015 in favor of Media.DetectedVideoCodecHash
@@ -19784,7 +19812,11 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 
 <histogram name="Media.DetectedVideoCodecHash" enum="FFmpegCodecHashes">
   <owner>jrummell@chromium.org</owner>
-  <summary>Video codec used in HTML5 media.</summary>
+  <summary>
+    Video codec used in HTML5 media. Reporting corrected in M51 to include all
+    detected video codecs, not just those up to and including the first one
+    supported.
+  </summary>
 </histogram>
 
 <histogram name="Media.DevicePermissionActions" enum="DevicePermissionActions">