Fix FakeSigninManager::SignOut

FakeSigninManager::SignOut was never updated to clear the account id on
SignOut. This result in calls to FakeSigninManager::IsAuthenticated
returning true after even after a SignOut.

BUG=

Review URL: https://codereview.chromium.org/980583003

Cr-Commit-Position: refs/heads/master@{#319077}

chrome/browser/signin/fake_signin_manager.cc

@@ -89,8 +89,8 @@ void FakeSigninManager::SignOut(
   set_auth_in_progress(std::string());
   set_password(std::string());
   const std::string account_id = GetAuthenticatedAccountId();
-  const std::string username = authenticated_username_;
-  authenticated_username_.clear();
+  const std::string username = GetAuthenticatedUsername();
+  ClearAuthenticatedUsername();
 
   FOR_EACH_OBSERVER(SigninManagerBase::Observer, observer_list_,
                     GoogleSignedOut(account_id, username));