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..36bc91b463849c11cb84e77ec18962183957dae4 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">

[xhwang, 2017/04/27 16:32:50]

OOC, is there any reason you can't just use enum="VideoCodec" here?

[kqyang, 2017/04/27 18:12:53]

Unfortunately the enums used by SRC and MSE are different:
https://cs.chromium.org/chromium/src/media/base/video_codecs.h?rcl=e052a49fd7...
https://cs.chromium.org/chromium/src/media/filters/stream_parser_factory.cc?r...

[xhwang, 2017/04/27 19:00:53]

hmm... good find, I didn't know about this

wolenetz: Do you know the history about MSECodec? Does it make sense to convert
this to VideoCodec and AudioCodec so we only have one set of codec UMA enums?
Currently we have MP3 listed as a bucket in Media.MSE.VideoCodec, which seems
odd.

+  <owner>kqyang@chromium.org</owner>

[xhwang, 2017/04/27 16:32:50]

The new recommendation is to use media-dev@chromium.org as the owner.

[kqyang, 2017/04/27 18:12:53]

Done.

+  <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>kqyang@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.
@@ -28628,6 +28644,20 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>Video codec used in HTML5 media.</summary>
 </histogram>
 
+<histogram name="Media.VideoCodec.MP4" enum="VideoCodec">
+  <owner>kqyang@chromium.org</owner>
+  <summary>
+    Video codec used in HTML5 media if the media container is MP4.

[xhwang, 2017/04/27 16:32:50]

This makes people think that it's for all HTML5 playback, including MSE and SRC,
but from the implementation this is only for SRC. We should probably either add
"SRC" into this UMA name, or if we prefer the current name, report to this UMA
from all demuxers.

[kqyang, 2017/04/27 18:12:53]

I think we want to track MSE and SRC separately.

I can change the name here to Media.SRC.VideoCodec.MP4/WebM, but it will make
the names inconsistent to the existing "Media.VideoCodec" definition. Are you ok
with that?

[xhwang, 2017/04/27 19:00:53]

Media.VideoCodec was probably added before MSE existed. We should probably
update it to include MSE as well.

[kqyang, 2017/04/27 21:07:59]

I did some code search. Surprisingly, this does not just affect one or two UMA
metrics, instead, it affects more than >15 metrics. You can search for
"UMA_HISTOGRAM" in "src/media/filters/ffmpeg_demuxer.cc", none of these metrics
track MSE playbacks.

We could fix them by either renaming the metric names to something like:
Media.SRC.VideoXxx or extends them to cover MSE as well (this will take some
time). In any case, I don't think it is a good idea to fix them in this CL. I
have opened a separate bug to track this: crbug/716183

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