Add an XPC service to handle alert notifications on mac

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index bf9083b98606458a60a5e68b02ddc51b71cac202..b88829b1c3ded834b363ac7e10e10ba14953b1c4 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -636,6 +636,11 @@ group("both_gn_and_gyp") {
     deps += [
       "//breakpad:crash_inspector",
       "//breakpad:dump_syms",
+
+      # XPC service is in developement and guarded against the
+      # enable_xpc_notifications, in the meantime however we still
+      # need compile coverage in the bots.
+      "//chrome/browser/ui/cocoa/notifications:alert_notification_xpc_service",
       "//third_party/apple_sample_code",
       "//third_party/molokocacao",
     ]