Encrypted Media: Add UMA for system code.

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 cb3c2d4fea972a943c0f628c085be47302bebbf7..a426a40d971b76d4c6b894583b376757de3cb743 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -11305,6 +11305,11 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <summary>KeyError event count using an unknown key system.</summary>
 </histogram>
 
+<histogram name="Media.EME.Unknown.SystemCode" units="system code">

[Ilya Sherman, 2014/09/11 05:15:29]

For these histograms, it would be more useful to associate an enum that
translates system codes into something that humans might understand, rather than
just stating that these are system codes and requiring users to go digging for
the mapping.

+  <owner>xhwang@chromium.org</owner>
+  <summary>System code count using an unknown key system.</summary>
+</histogram>
+
 <histogram name="Media.EME.Widevine.addKey" enum="MediaKeyException">
   <owner>xhwang@chromium.org</owner>
   <summary>addKey result using the Widevine key system.</summary>
@@ -11343,6 +11348,11 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <summary>KeyError event count using the Widevine key system.</summary>
 </histogram>
 
+<histogram name="Media.EME.Widevine.SystemCode" units="system code">
+  <owner>xhwang@chromium.org</owner>
+  <summary>System code count using the Widevine key system.</summary>
+</histogram>
+
 <histogram name="Media.FallbackHardwareAudioBitsPerChannel">
   <owner>dalecurtis@chromium.org</owner>
   <summary>