Cleanup: Pass std::string as const reference if possible

components/invalidation/impl/gcm_network_channel_unittest.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/run_loop.h"
 #include "base/strings/string_split.h"
 #include "base/strings/string_util.h"
 #include "base/thread_task_runner_handle.h"
 #include "components/invalidation/impl/gcm_network_channel.h"
 #include "google_apis/gaia/google_service_auth_error.h"
@@ skipping 152 matching lines @@
   static bool Base64DecodeURLSafe(const std::string& input,
                                   std::string* output) {
     return GCMNetworkChannel::Base64DecodeURLSafe(input, output);
   }
 
   void OnNetworkChannelStateChanged(
       InvalidatorState invalidator_state) override {
     last_invalidator_state_ = invalidator_state;
   }
 
-  void OnIncomingMessage(std::string incoming_message) {
-  }
+  void OnIncomingMessage(std::string /* incoming_message */) {}
 
   GCMNetworkChannel* network_channel() {
     return gcm_network_channel_.get();
   }
 
   TestGCMNetworkChannelDelegate* delegate() {
     return delegate_;
   }
 
   int url_fetchers_created_count() {
@@ skipping 320 matching lines @@
   delegate()->request_token_callback.Run(
       GoogleServiceAuthError::AuthErrorNone(), "access.token");
   RunLoopUntilIdle();
   EXPECT_EQ(url_fetchers_created_count(), 3);
   // Echo_token should be from second message.
   EXPECT_EQ("echo.token", get_last_echo_token());
 }
 #endif
 
 }  // namespace syncer