Adding Type Alias to Push Notification Infra

In this CL, the PrefereneceMap type alias was defined to make the push
notificaition service's code more readable.

Bug: 1369616
Change-Id: I703091c13a0f18ff91756a59eca9c434103caf81
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3949351
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Daniel White <danieltwhite@google.com>
Cr-Commit-Position: refs/heads/main@{#1058715}
diff --git a/ios/chrome/browser/push_notification/push_notification_service.h b/ios/chrome/browser/push_notification/push_notification_service.h
index 60465879..c5aaa0e8 100644
--- a/ios/chrome/browser/push_notification/push_notification_service.h
+++ b/ios/chrome/browser/push_notification/push_notification_service.h
@@ -19,6 +19,7 @@
 class PushNotificationService {
  public:
   using CompletionHandler = void (^)(NSError* error);
+  using PreferenceMap = NSDictionary<NSString*, NSNumber*>*;
 
   PushNotificationService();
   virtual ~PushNotificationService();
@@ -38,7 +39,7 @@
   // notification server.
   void UpdateFeaturePushNotificationPreferences(
       NSString* account_id,
-      NSDictionary<NSString*, NSNumber*>* preference_map,
+      PreferenceMap preference_map,
       CompletionHandler completion_handler) {}
 
   // Registers each PushNotificationClient's prefs. Each