Added User Timing UMA metrics to the histograms definition

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 abba94fab24fb5c0449b95e18e30729c7a874f69..b9473f5b1476e0eed4342d80ae94b9d69df045ed 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -13280,6 +13280,22 @@ other types of suffix sets.
   </summary>
 </histogram>
 
+<histogram name="PLT.UserTiming_Mark" units="milliseconds">
+  <summary>
+    This time is based on the User Timing spec and measures the time from
+    Navigation Timing navigationStart until the point where the page called
+    performance.mark().
+  </summary>
+</histogram>
+
+<histogram name="PLT.UserTiming_MeasureDuration" units="milliseconds">
+  <summary>
+    This time is based on the User Timing spec and reports the time between two
+    arbitrary points defined by the page being loaded and directly matches the
+    measurement exposed by performance.measure().
+  </summary>
+</histogram>
+
 <histogram name="Plugin.PpapiBrokerLoadResult" enum="PluginLoadResult">
   <summary>The result from an attempt to load a PPAPI broker.</summary>
 </histogram>