Cleanup: Pass std::string as const reference from content/

content/browser/device_monitor_mac.mm

 // Copyright (c) 2012 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 "content/browser/device_monitor_mac.h"
 
 #import <QTKit/QTKit.h>
 
 #include <set>
 
@@ skipping 14 matching lines @@
 class DeviceInfo {
  public:
   enum DeviceType {
     kAudio,
     kVideo,
     kMuxed,
     kUnknown,
     kInvalid
   };
 
-  DeviceInfo(std::string unique_id, DeviceType type)
+  DeviceInfo(const std::string& unique_id, DeviceType type)
       : unique_id_(unique_id), type_(type) {}
 
   // Operator== is needed here to use this class in a std::find. A given
   // |unique_id_| always has the same |type_| so for comparison purposes the
   // latter can be safely ignored.
   bool operator==(const DeviceInfo& device) const {
     return unique_id_ == device.unique_id_;
   }
 
   const std::string& unique_id() const { return unique_id_; }
@@ skipping 506 matching lines @@
 }
 
 void DeviceMonitorMac::NotifyDeviceChanged(
     base::SystemMonitor::DeviceType type) {
   DCHECK(thread_checker_.CalledOnValidThread());
   // TODO(xians): Remove the global variable for SystemMonitor.
   base::SystemMonitor::Get()->ProcessDevicesChanged(type);
 }
 
 }  // namespace content