Removing x-x509-user-cert mime handler.

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 8edba605650c584b134a7fddb92a1fb7122f4d70..b46168ca2226007c80a87a41aa0b0c7b6a616173 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -51887,6 +51887,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="UserCert.ContentDisposition" enum="UserCertContentDisposition">
+  <owner>rsleevi@chromium.org</owner>
+  <owner>svaldez@chromium.org</owner>
+  <summary>
+    Distribution of Content-Disposition headers sent with x-x509-user-cert
+    content types.
+  </summary>
+</histogram>
+
 <histogram name="UserImage.ChangeChoice" enum="ChromeOSUserImageId">
   <owner>achuith@chromium.org</owner>
   <summary>
@@ -79295,6 +79304,11 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="1" label="Was used in draw"/>
 </enum>
 
+<enum name="UserCertContentDisposition" type="int">
+  <int value="0" label="No Content-Disposition"/>
+  <int value="1" label="Content-Disposition"/>
+</enum>
+
 <enum name="UserChannels" type="int">
   <int value="-1" label="Unknown"/>
   <int value="0" label="Canary"/>