Add IsAuthenticated() method to SigninManager.

components/signin/core/browser/signin_manager_base.cc

Index: components/signin/core/browser/signin_manager_base.cc
diff --git a/components/signin/core/browser/signin_manager_base.cc b/components/signin/core/browser/signin_manager_base.cc
index 52172e8f81bba051a7f8880b08974623035116a9..770cf7af25e900e8057580ded00d952e0b61743c 100644
--- a/components/signin/core/browser/signin_manager_base.cc
+++ b/components/signin/core/browser/signin_manager_base.cc
@@ -96,6 +96,10 @@ void SigninManagerBase::clear_authenticated_username() {
   authenticated_username_.clear();
 }
 
+bool SigninManagerBase::IsAuthenticated() const {
+  return !GetAuthenticatedAccountId().empty();
+}
+
 bool SigninManagerBase::AuthInProgress() const {
   // SigninManagerBase never kicks off auth processes itself.
   return false;