Deprecate PLT UserTiming histograms that are no longer needed.

We will stop logging these and other PLT.* histograms in M56.
For the time being, we're deprecating them.

BUG=384330

Review-Url: https://codereview.chromium.org/2348463002
Cr-Commit-Position: refs/heads/master@{#418913}

tools/metrics/histograms/histograms.xml

@@ -44398,6 +44398,9 @@ 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
@@ -44407,6 +44410,9 @@ 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