Add warning message for empty robustness.

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 b61a47929def396f138a0f1f311cb5e9a13fdfaa..51f2accdb840ce155e371fa91d869868170b7121 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -17524,6 +17524,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>System code count using the Widevine key system.</summary>
 </histogram>
 
+<histogram name="Media.EME.Widevine.VideoCapability.HasEmptyRobustness"
+    enum="BooleanEmpty">
+  <owner>xhwang@chromium.org</owner>
+  <summary>
+    Whether the robustness level of any video capability in the supported
+    configuration passed into requestMediaKeySystemAccess() is empty. This is
+    specific to the Widevine key system.
+  </summary>
+</histogram>
+
 <histogram name="Media.FallbackHardwareAudioBitsPerChannel">
   <owner>dalecurtis@chromium.org</owner>
   <summary>
@@ -52740,6 +52750,11 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="1" label="Duplicate"/>
 </enum>
 
+<enum name="BooleanEmpty" type="int">
+  <int value="0" label="Not Empty"/>
+  <int value="1" label="Empty"/>
+</enum>
+
 <enum name="BooleanEnabled" type="int">
   <int value="0" label="Disabled"/>
   <int value="1" label="Enabled"/>