Initialize SigninManager for ios AccountConsistencyServiceTest

The (Fake)SigninManager should always be initialized before using. This
is made more critical by upcoming changes* that will cause the test to
fail if initialization is not performed.

*https://chromium-review.googlesource.com/c/chromium/src/+/1154985

Bug: 806781
Change-Id: I06989a895f86b3ca9b1b84760b54ecafdaafdb03
Reviewed-on: https://chromium-review.googlesource.com/1160643
Commit-Queue: Chrome Cunningham (In Paris) <chcunningham@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580204}

components/signin/ios/browser/account_consistency_service_unittest.mm

@@ -149,9 +149,10 @@ class AccountConsistencyServiceTest : public PlatformTest {
     web_view_load_expection_count_ = 0;
     gaia_cookie_manager_service_.reset(new MockGaiaCookieManagerService());
     signin_client_.reset(new TestSigninClient(&prefs_));
+    account_tracker_service_.Initialize(signin_client_.get());
     signin_manager_.reset(new FakeSigninManager(
         signin_client_.get(), nullptr, &account_tracker_service_, nullptr));
-    account_tracker_service_.Initialize(signin_client_.get());
+    signin_manager_->Initialize(nullptr);
     settings_map_ = new HostContentSettingsMap(
         &prefs_, false /* incognito_profile */, false /* guest_profile */,
         false /* store_last_modified */);