Deprecate PLT UserTiming histograms that are no longer needed.

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 5ef617d9dfbd1d54d00646242fdddef5839a9407..81f7cff3289950c124992f12b8e982da568f94e8 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -44065,6 +44065,9 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 </histogram>
 
 <histogram name="PLT.UserTiming_Mark" units="ms">
+  <obsolete>
+    Deprecated as of 09/2016.
+  </obsolete>
   <owner>pmeenan@chromium.org</owner>
   <summary>
     This time is based on the User Timing spec and measures the time from
@@ -44074,6 +44077,9 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 </histogram>
 
 <histogram name="PLT.UserTiming_MeasureDuration" units="ms">
+  <obsolete>
+    Deprecated as of 09/2016.
+  </obsolete>
   <owner>pmeenan@chromium.org</owner>
   <summary>
     This time is based on the User Timing spec and reports the time between two