media: Add incognito EME playback UMA

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 b60418472cd77735b8f6d61532c05dfa178c0173..86d4c4c183913f9e04debbd294fdda67fbf8141b 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -26930,6 +26930,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Media.EME.IsIncognito" enum="BooleanIncognito">
+  <owner>xhwang@chromium.org</owner>
+  <summary>
+    Whether a given WebMediaPlayer instance, with a CDM attached, is played in
+    an incognito window or in ChromeOS guest mode. Players that never started
+    playback are excluded. Recorded once at time of player destruction.
+  </summary>
+</histogram>
+
 <histogram name="Media.EME.KeySystemSupport.Widevine"
     enum="MediaKeySystemSupportStatus">
   <obsolete>
@@ -84683,6 +84692,11 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="1" label="inappropriate_fallback alert"/>
 </enum>
 
+<enum name="BooleanIncognito" type="int">
+  <int value="0" label="Not Incognito"/>
+  <int value="1" label="Incognito"/>
+</enum>
+
 <enum name="BooleanInForeground" type="int">
   <int value="0" label="Background"/>
   <int value="1" label="Foreground"/>