@Override
    public RequestHandler build() {
      OkHttpClient client = builder.build();
      return new OkHttpRequestHandler(client, rateLimitExecutorService);
    }
Gets a reference to the OkHttpClient.Builder used to build the OkHttpRequestHandler's
internal OkHttpClient. This allows you to fully customize the OkHttpClient that the resulting
OkHttpRequestHandler will make HTTP requests through.

@return OkHttpClient.Builder that will produce the OkHttpClient used by the
    OkHttpRequestHandler built by this.
src/main/java/com/google/maps/OkHttpRequestHandler.java