media: Add UMA and trace events for CDM creation.

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 e434ba083dc00c6aca62a0beb903c9ff5e32e86c..034ef1962cb72eff4699543a3a4193c630ad99ac 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -16026,6 +16026,13 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>cancelKeyRequest result using the Widevine key system.</summary>
 </histogram>
 
+<histogram name="Media.EME.Widevine.CreateCdm" units="milliseconds">

[ddorwin, 2015/07/20 21:35:21]

Other key systems will be logged but not have a description, right? Is that
acceptable? I don't want to unnecessarily clutter this, but I don't want to
orphan values either if that's not acceptable.

[xhwang, 2015/07/20 22:11:57]

Correct. Widevine is the only case that we are interested in.

I *think* it's acceptable. The data for other key systems will be kept on the
server but not displayed. If we ever want to see those data, we can add an entry
in this file. +asvitkine to confirm.

[Alexei Svitkine (slow), 2015/07/21 17:13:48]

From UMA point of view, this is not ideal. Preferably, we would like to be able
to decode all histogram hashes on the server side. So if we know what the others
are already, it's best to document them in this CL. You can use the
histogram_suffixes construct to avoid too much duplication in the XML (see the
comments at the top of the file for some examples of how to).

Alternatively, if you really don't care about the data for the other ones, then
the other thing you could do is add a conditional to the code to only log this
for Widevine.

[xhwang, 2015/07/21 22:14:22]

Done. 

histogram_suffixes doesn't fit cleanly in this case. I do want to keep
Media.EME.Widevine.* pattern, which most existing EME metrics use today. The
ordering="prefix" trick doesn't work as well as Media.EME. has two dots....

+  <owner>xhwang@chromium.org</owner>
+  <summary>
+    The time it takes to create the CDM instance for the Widevine key system.
+  </summary>
+</histogram>
+
 <histogram name="Media.EME.Widevine.generateKeyRequest"
     enum="MediaKeyException">
   <owner>xhwang@chromium.org</owner>