Coverity: Fix a pass-by-value.

chrome/browser/content_settings/content_settings_observable_provider.h

 // 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.
 
 #ifndef CHROME_BROWSER_CONTENT_SETTINGS_CONTENT_SETTINGS_OBSERVABLE_PROVIDER_H_
 #define CHROME_BROWSER_CONTENT_SETTINGS_CONTENT_SETTINGS_OBSERVABLE_PROVIDER_H_
 
 #include <string>
 
 #include "base/observer_list.h"
 #include "chrome/browser/content_settings/content_settings_provider.h"
 #include "chrome/browser/content_settings/content_settings_observer.h"
 #include "chrome/common/content_settings_pattern.h"
 
 namespace content_settings {
 
 class ObservableProvider : public ProviderInterface {
  public:
   ObservableProvider();
   virtual ~ObservableProvider();
 
   void AddObserver(Observer* observer);
   void RemoveObserver(Observer* observer);
 
  protected:
   void NotifyObservers(const ContentSettingsPattern& primary_pattern,
                        const ContentSettingsPattern& secondary_pattern,
                        ContentSettingsType content_type,
-                       std::string resource_identifier);
+                       const std::string& resource_identifier);
   void RemoveAllObservers();
 
  private:
   ObserverList<Observer, true> observer_list_;
 };
 
 }  // namespace content_settings
 
 #endif  // CHROME_BROWSER_CONTENT_SETTINGS_CONTENT_SETTINGS_OBSERVABLE_PROVIDER_H_