Update docs for GoogleServiceAuthError::State::USER_NOT_SIGNED_UP

|GoogleServiceAuthError::State::USER_NOT_SIGNED_UP| is now thrown iff
|OAuth2AccessTokenManager::Delegate::HasRefreshToken| is |false|.

Update the documentation for
|GoogleServiceAuthError::State::USER_NOT_SIGNED_UP| to reflect this.

Change-Id: I5f5f4ea9d6706254092074f0ec48b9c4fdde6c88
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2386798Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Kush Sinha <sinhak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804916}

google_apis/gaia/google_service_auth_error.h

@@ -29,7 +29,7 @@ class GoogleServiceAuthError {
     // cached credentials have expired.
     INVALID_GAIA_CREDENTIALS = 1,
 
-    // The GAIA user is not authorized to use the service.
+    // Chrome does not have credentials (tokens) for this account.
     USER_NOT_SIGNED_UP = 2,
 
     // Could not connect to server to verify credentials. This could be in