Currently, IdentityGetAuthTokenFunction is the only client of the Mojo Identity Service in the codebase. We want to have it continue being a client so that the Identity Service continues being exercised in production code. However, we have also had to introduce it using the C++ IdentityManager as well in order to eliminate its usage of legacy signin classes (e.g., SigninManager and ProfileOAuth2TokenService).

The situation of it simultaneously using both the Mojo and C++ IdentityManager classes is obviously undesirable long-term. When we build out a version of IdentityManager that is a client library of the Identity Service, we should move IdentityGetAuthTokenFunction to use *only* that IdentityManager.