Introduce ServiceWorker.EventDispatchingDelay UMA

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 026d1e90ac0583365d77d13e75c2cae26eb883bf..5b9aeafe83175374867af3604260803a530affab 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -52673,6 +52673,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="ServiceWorker.FetchEvent.DispatchingDelay" units="ms">
+  <owner>horo@chromium.org</owner>
+  <summary>
+    The time taken between sending a FetchEvent IPC from the browser process to
+    a Service Worker and executing the fetch event handler in the Service
+    Worker.
+  </summary>
+</histogram>
+
 <histogram name="ServiceWorker.FetchEvent.Fallback.Time" units="ms">
   <owner>jeremyarcher@chromium.org</owner>
   <summary>
@@ -102739,7 +102748,11 @@ To add a new entry, add it with any value and run test to compute valid value.
 </histogram_suffixes>
 
 <histogram_suffixes name="ServiceWorkerSpecialApps" separator=".">
+  <suffix name="docs" label="Custom histogram for Google Docs and Drive"/>
+  <suffix name="gmail" label="Custom histogram for GMail"/>
   <suffix name="inbox" label="Custom histogram for Inbox"/>
+  <suffix name="ntp" label="Custom histogram for New Tab Page"/>
+  <suffix name="plus" label="Custom histogram for Google+"/>
   <affected-histogram
       name="PageLoad.Clients.ServiceWorker.DocumentTiming.NavigationToDOMContentLoadedEventFired"/>
   <affected-histogram
@@ -102748,6 +102761,7 @@ To add a new entry, add it with any value and run test to compute valid value.
       name="PageLoad.Clients.ServiceWorker.PaintTiming.NavigationToFirstContentfulPaint"/>
   <affected-histogram
       name="PageLoad.Clients.ServiceWorker.PaintTiming.ParseStartToFirstContentfulPaint"/>
+  <affected-histogram name="ServiceWorker.FetchEvent.DispatchingDelay"/>
 </histogram_suffixes>
 
 <histogram_suffixes name="SessionRestoreTabCountMemoryPressure">