[Extensions Service Worker] Add histogram for event dispatch.

This CL adds the histogram
Extensions.Events.DispatchWithServiceWorkerBackground when an extension
event is dispatched to the background service worker of an extension.

Bug: None
Change-Id: I6a119dfb4d39b518201c8cf166e99a2f4595dac1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2441949
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: David Bertoni <dbertoni@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812576}

extensions/browser/event_router.cc

@@ -11,6 +11,7 @@
 
 #include "base/atomic_sequence_num.h"
 #include "base/bind.h"
+#include "base/metrics/histogram_functions.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/stl_util.h"
 #include "base/task/post_task.h"
@@ -831,6 +832,10 @@ void EventRouter::ReportEvent(events::HistogramValue histogram_value,
           "Extensions.Events.DispatchWithRunningEventPage", histogram_value,
           events::ENUM_BOUNDARY);
     }
+  } else if (BackgroundInfo::IsServiceWorkerBased(extension)) {
+    base::UmaHistogramEnumeration(
+        "Extensions.Events.DispatchWithServiceWorkerBackground",
+        histogram_value, events::ENUM_BOUNDARY);
   }
 }
 

tools/metrics/histograms/histograms_xml/extensions/histograms.xml

@@ -1079,6 +1079,17 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   </summary>
 </histogram>
 
+<histogram name="Extensions.Events.DispatchWithServiceWorkerBackground"
+    enum="ExtensionEvents" expires_after="2021-10-01">
+  <owner>lazyboy@chromium.org</owner>
+  <owner>dbertoni@chromium.org</owner>
+  <summary>
+    Recorded every time an event is dispatched to a service worker based
+    extension background context. When this metric is recorded,
+    Extensions.Events.Dispatch will also be recorded.
+  </summary>
+</histogram>
+
 <histogram name="Extensions.Events.DispatchWithSuspendedEventPage"
     enum="ExtensionEvents" expires_after="never">
 <!-- expires-never: Monitoring core extensions platform behavior. -->