code: public <T extends AbstractGoogleJsonClient.Builder> T fromCredential(T builder, Credential credential) {
      Preconditions.checkNotNull(builder);
      Preconditions.checkNotNull(credential);
      return prepareBuilder(builder, credential, null);
    }
docs: Prepare an AbstractGoogleJsonClient.Builder using a credential.
  
  @param builder The builder to be prepared.
  @param credential The credential to be used for requests.
  @return The passed in builder, for easy chaining.