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

device/bluetooth/bluetooth_adapter_profile_chromeos_unittest.cc

 // Copyright 2015 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/bind.h"
 #include "base/message_loop/message_loop.h"
 #include "chromeos/dbus/bluetooth_profile_service_provider.h"
 #include "chromeos/dbus/dbus_thread_manager.h"
 #include "chromeos/dbus/fake_bluetooth_adapter_client.h"
 #include "chromeos/dbus/fake_bluetooth_agent_manager_client.h"
@@ skipping 56 matching lines @@
     DBusThreadManager::Shutdown();
   }
 
   void AdapterCallback(scoped_refptr<BluetoothAdapter> adapter) {
     adapter_ = adapter;
   }
 
   class FakeDelegate
       : public chromeos::BluetoothProfileServiceProvider::Delegate {
    public:
-    FakeDelegate(std::string device_path) : connections_(0) {
+    FakeDelegate(const std::string& device_path) : connections_(0) {
       device_path_ = dbus::ObjectPath(device_path);
     }
 
     // BluetoothProfileServiceProvider::Delegate:
     void Released() override {
       // noop
     }
 
     void NewConnection(
         const dbus::ObjectPath& device_path,
@@ skipping 286 matching lines @@
                  base::Unretained(this)));
 
   message_loop_.RunUntilIdle();
 
   EXPECT_FALSE(profile_user_ptr_);
   EXPECT_EQ(0U, success_callback_count_);
   EXPECT_EQ(2U, error_callback_count_);
 }
 
 }  // namespace chromeos