Add UMA metric to track the time saved on making events passive before pageload

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 a3bafdbe5d65bba604612ccc74980a44b9ceeded..22392eb1c17a18b16ff67b5f807f238474265493 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -12418,6 +12418,25 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Event.Touch.TouchLatencyAfterPageLoad" units="microseconds">
+  <owner>tdresser@chromium.org</owner>
+  <summary>
+    Time between when a touch event was generated and the event was processed.
+    Recorded only for touch start events and the first touchmove events per
+    scroll that occur after the page is fully loaded.
+  </summary>
+</histogram>
+
+<histogram name="Event.Touch.TouchLatencyBeforePageLoad" units="microseconds">
+  <owner>tdresser@chromium.org</owner>
+  <summary>
+    Time between when a touch event was generated and the event was processed.
+    Recorded only for touch start events and the first touchmove events per
+    scroll that occur before the page is fully loaded. This histogram tracks the
+    benefit of forcing passive event listeners before the page is fully loaded.
+  </summary>
+</histogram>
+
 <histogram name="Event.TouchDuration" units="ms">
   <owner>kuscher@google.com</owner>
   <owner>rbyers@chromium.org</owner>