Cleanup: Pass std::string as const reference from google_apis/ and components/gcm_driver/

google_apis/gaia/oauth2_token_service_test_util.cc

Index: google_apis/gaia/oauth2_token_service_test_util.cc
diff --git a/google_apis/gaia/oauth2_token_service_test_util.cc b/google_apis/gaia/oauth2_token_service_test_util.cc
index 6dbed5f8974903b75d09bc031db3db291442b396..597d61d6040f8b84a4fc7feff571abceb74eefaf 100644
--- a/google_apis/gaia/oauth2_token_service_test_util.cc
+++ b/google_apis/gaia/oauth2_token_service_test_util.cc
@@ -15,7 +15,7 @@ const char kValidTokenResponse[] =
     "}";
 }
 
-std::string GetValidTokenResponse(std::string token, int expiration) {
+std::string GetValidTokenResponse(const std::string& token, int expiration) {
   return base::StringPrintf(kValidTokenResponse, token.c_str(), expiration);
 }