[iOS] Set active_cookie_manager_requests_for_testing_ to zero

AccountConsistencyService::active_cookie_manager_requests_for_testing_
is not set to 0.
AccountConsistencyServiceTest fails.

Related to crrev.com/c/2346664

Fixed: 1123568
Change-Id: If9842d57daef2bcc4e3c365ada38d5668ddbcc69
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2385516
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Commit-Queue: Nohemi Fernandez <fernandex@chromium.org>
Auto-Submit: Jérôme Lebel <jlebel@chromium.org>
Reviewed-by: Nohemi Fernandez <fernandex@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803164}

components/signin/ios/browser/account_consistency_service.mm

@@ -178,7 +178,8 @@ AccountConsistencyService::AccountConsistencyService(
       prefs_(prefs),
       account_reconcilor_(account_reconcilor),
       cookie_settings_(cookie_settings),
-      identity_manager_(identity_manager) {
+      identity_manager_(identity_manager),
+      active_cookie_manager_requests_for_testing_(0) {
   identity_manager_->AddObserver(this);
   LoadFromPrefs();
   if (identity_manager_->HasPrimaryAccount()) {