Removed DefaultBrowser.SetDefaultAsyncDuration histogram

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 5e334167a6657c475f9f07d0e104dc6233e13b05..2c1fe9c9da8142626e408c8362d278ee3cbe5c03 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -7343,6 +7343,9 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 </histogram>
 
 <histogram name="DefaultBrowser.SetDefaultAsyncDuration" units="ms">
+  <obsolete>
+    Deprecated 2016/03. The async set-as-default experiments are finished.
+  </obsolete>
   <owner>pmonette@chromium.org</owner>
   <summary>
     How long it took to set Chrome as the default browser asynchronously in
@@ -84204,6 +84207,9 @@ To add a new entry, add it with any value and run test to compute valid value.
 </histogram_suffixes>
 
 <histogram_suffixes name="AttemptResultCode">
+  <obsolete>
+    Deprecated 2016/03. The async set-as-default experiments are finished.
+  </obsolete>
   <suffix name="Success"/>
   <suffix name="Failure"/>
   <suffix name="Abandoned"/>