[ios] Add method to initialize Firebase to AppDistributionProvider

As a first step towards using Firebase via the internal framework,
and thus removing the dependency upstream on Firebase, add a method
to AppDistributionProvider to initialize Firebase.

Bug: 1049042
Change-Id: Ifbbb211b844eeb564462586755f66913a01d5bb6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2057750
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Peter Lee <pkl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#741687}

ios/public/provider/chrome/browser/distribution/app_distribution_provider.h

@@ -30,6 +30,11 @@ class AppDistributionProvider {
   // Cancels any pending distribution notifications.
   virtual void CancelDistributionNotifications();
 
+  // Initializes Firebase for installation attribution purpose. |install_date|
+  // is used to detect "legacy" users that installed Chrome before Firebase
+  // was integrated and thus should not have Firebase enabled.
+  virtual void InitializeFirebase(int64_t install_date, bool is_first_run);
+
   // Returns whether user who installed Chrome on |install_date| predates
   // integration with Firebase for installation attribution.
   virtual bool IsPreFirebaseLegacyUser(int64_t install_date);

ios/public/provider/chrome/browser/distribution/app_distribution_provider.mm

@@ -22,6 +22,9 @@ void AppDistributionProvider::ScheduleDistributionNotifications(
 
 void AppDistributionProvider::CancelDistributionNotifications() {}
 
+void AppDistributionProvider::InitializeFirebase(int64_t install_date,
+                                                 bool is_first_run) {}
+
 bool AppDistributionProvider::IsPreFirebaseLegacyUser(int64_t install_date) {
   return false;
 }