Add UMA histogram Arc.AppsInstalledAtStartup to measure the number of ARC apps installed.

chrome/browser/ui/app_list/arc/arc_app_list_prefs.cc

Index: chrome/browser/ui/app_list/arc/arc_app_list_prefs.cc
diff --git a/chrome/browser/ui/app_list/arc/arc_app_list_prefs.cc b/chrome/browser/ui/app_list/arc/arc_app_list_prefs.cc
index 687d71419f486160e4183ef2608f98156a0e5f40..7258c8bc70761da9558a2ed485fe1a8e9862f709 100644
--- a/chrome/browser/ui/app_list/arc/arc_app_list_prefs.cc
+++ b/chrome/browser/ui/app_list/arc/arc_app_list_prefs.cc
@@ -10,6 +10,7 @@
 
 #include "base/files/file_util.h"
 #include "base/macros.h"
+#include "base/metrics/histogram_macros.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/task_runner_util.h"
 #include "chrome/browser/ui/app_list/arc/arc_app_list_prefs_factory.h"
@@ -403,6 +404,11 @@ void ArcAppListPrefs::OnAppListRefreshed(mojo::Array<arc::AppInfoPtr> apps) {
     if (!ready_apps_.count(app_id))
       RemoveApp(app_id);
   }
+
+  if (!is_initialized_) {
+    is_initialized_ = true;
+    UMA_HISTOGRAM_COUNTS_1000("Arc.AppsInstalledAtStartup", ready_apps_.size());
+  }
 }
 
 void ArcAppListPrefs::OnAppAdded(arc::AppInfoPtr app) {