Introduce CryptAuthClient, a class capable of performing all CryptAuth APIs.

components/proximity_auth/cryptauth/cryptauth_api_call_flow.cc

Index: components/proximity_auth/cryptauth/cryptauth_api_call_flow.cc
diff --git a/components/proximity_auth/cryptauth/cryptauth_api_call_flow.cc b/components/proximity_auth/cryptauth/cryptauth_api_call_flow.cc
index 05acd5c19ebf84b7ab658760e4870b4ecc50b367..399b949200f1d311b3c16332abe2716ad6a4c7d9 100644
--- a/components/proximity_auth/cryptauth/cryptauth_api_call_flow.cc
+++ b/components/proximity_auth/cryptauth/cryptauth_api_call_flow.cc
@@ -27,8 +27,8 @@ CryptAuthApiCallFlow::~CryptAuthApiCallFlow() {
 void CryptAuthApiCallFlow::Start(net::URLRequestContextGetter* context,
                                  const std::string& access_token,
                                  const std::string& serialized_request,
-                                 ResultCallback result_callback,
-                                 ErrorCallback error_callback) {
+                                 const ResultCallback& result_callback,
+                                 const ErrorCallback& error_callback) {
   serialized_request_ = serialized_request;
   result_callback_ = result_callback;
   error_callback_ = error_callback;