Include Service worker in "Extensions.BackgroundPageType" histogram.

This CL adds SERVICE_WORKER to BackgroundPageType enum that is used
to record "Extensions.BackgroundPageType" histogram for installed
extensions from installed_loader (on startup).

This change also fixes typo in comment
"Extension.BackgroundPageType"

Bug: None
Change-Id: I6f2364b7d3af4ae388750f44e6d3e76b7d707f0a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2164250Reviewed-by: David Bertoni <dbertoni@chromium.org>
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#762649}

chrome/browser/extensions/installed_loader.cc

@@ -65,11 +65,12 @@ enum ManifestReloadReason {
   NUM_MANIFEST_RELOAD_REASONS
 };
 
-// Used in histogram Extension.BackgroundPageType.
+// Used in histogram Extensions.BackgroundPageType.
 enum BackgroundPageType {
   NO_BACKGROUND_PAGE = 0,
   BACKGROUND_PAGE_PERSISTENT,
   EVENT_PAGE,
+  SERVICE_WORKER,
 
   // New enum values must go above here.
   NUM_BACKGROUND_PAGE_TYPES
@@ -127,6 +128,8 @@ BackgroundPageType GetBackgroundPageType(const Extension* extension) {
     return NO_BACKGROUND_PAGE;
   if (BackgroundInfo::HasPersistentBackgroundPage(extension))
     return BACKGROUND_PAGE_PERSISTENT;
+  if (BackgroundInfo::IsServiceWorkerBased(extension))
+    return SERVICE_WORKER;
   return EVENT_PAGE;
 }
 

tools/metrics/histograms/enums.xml

@@ -20616,6 +20616,7 @@ Called by update_document_policy_enum.py.-->
   <int value="0" label="None"/>
   <int value="1" label="Persistent"/>
   <int value="2" label="Event Page"/>
+  <int value="3" label="Service worker"/>
 </enum>
 
 <enum name="ExtensionBubbleAction">