Change CredentialsProvider::TokenListener to use StatusOr<Token> (#945)

* Change CredentialsProvider::TokenListener to use StatusOr

Rather than a token plus error code/msg.

* Eliminate the concept of an invalid Token

Instead, we'll just use StatusOr<Token>. Note that unauthenticated
tokens are handled as a special case; they're created via:
  Token::Unauthenticated()
and are otherwise "valid", though attempting to retrieve the raw token
on one of these tokens will cause an assertion failure.

1 files changed, 3 insertions, 2 deletions

diff --git a/Firestore/core/src/firebase/firestore/auth/empty_credentials_provider.cc b/Firestore/core/src/firebase/firestore/auth/empty_credentials_provider.cc
index 0fa70c0..da4198d 100644
--- a/Firestore/core/src/firebase/firestore/auth/empty_credentials_provider.cc
+++ b/Firestore/core/src/firebase/firestore/auth/empty_credentials_provider.cc
@@ -26,8 +26,9 @@ void EmptyCredentialsProvider::GetToken(bool force_refresh,
                                         TokenListener completion) {
   UNUSED(force_refresh);
   if (completion) {
-    // Invalid token will force the GRPC fallback to use default settings.
-    completion(Token::Invalid(), FirestoreErrorCode::Ok, "");
+    // Unauthenticated token will force the GRPC fallback to use default
+    // settings.
+    completion(Token::Unauthenticated());
   }
 }