Remove support for the keygen tag

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 c72577d88e346d0705975ccd28bc595c607c52e3..2eef80572101a9a30cddbb1226eeeba955525067 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -7258,6 +7258,9 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 </histogram>
 
 <histogram name="ContentSettings.DefaultKeygenSetting" enum="ContentSetting">
+  <obsolete>
+    Support for keygen was removed.
+  </obsolete>
   <owner>svaldez@chromium.org</owner>
   <summary>
     The default setting for using key generation in forms at profile open.
@@ -69851,6 +69854,9 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 </histogram>
 
 <histogram name="UserCert.ContentDisposition" enum="UserCertContentDisposition">
+  <obsolete>
+    Removed in M57.
+  </obsolete>
   <owner>rsleevi@chromium.org</owner>
   <owner>svaldez@chromium.org</owner>
   <summary>
@@ -86965,7 +86971,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="883" label="V8Performance_Navigation_AttributeGetter"/>
   <int value="884" label="V8Performance_Memory_AttributeGetter"/>
   <int value="885" label="V8SharedWorker_WorkerStart_AttributeGetter"/>
-  <int value="886" label="HTMLKeygenElement"/>
+  <int value="886" label="OBSOLETE_HTMLKeygenElement"/>
   <int value="887" label="V8SVGElement_OffsetParent_AttributeGetter"/>
   <int value="888" label="V8SVGElement_OffsetTop_AttributeGetter"/>
   <int value="889" label="V8SVGElement_OffsetLeft_AttributeGetter"/>