Fixed coverity defects regarding pass-by-value

chrome/browser/extensions/shell_window_registry.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_EXTENSIONS_SHELL_WINDOW_REGISTRY_H_
 #define CHROME_BROWSER_EXTENSIONS_SHELL_WINDOW_REGISTRY_H_
 
 #include <set>
 
 #include "base/compiler_specific.h"
@@ skipping 40 matching lines @@
   // a convenience wrapper around ShellWindowRegistryFactory::GetForProfile.
   static ShellWindowRegistry* Get(Profile* profile);
 
   void AddShellWindow(ShellWindow* shell_window);
   void RemoveShellWindow(ShellWindow* shell_window);
 
   void AddObserver(Observer* observer);
   void RemoveObserver(Observer* observer);
 
   // Returns a set of windows owned by the application identified by app_id.
-  ShellWindowSet GetShellWindowsForApp(const std::string app_id) const;
+  ShellWindowSet GetShellWindowsForApp(const std::string& app_id) const;
   const ShellWindowSet& shell_windows() const { return shell_windows_; }
 
   // Helper functions to find shell windows with particular attributes.
   ShellWindow* GetShellWindowForRenderViewHost(
       content::RenderViewHost* render_view_host) const;
   ShellWindow* GetShellWindowForNativeWindow(gfx::NativeWindow window) const;
 
  private:
   class Factory : public ProfileKeyedServiceFactory {
    public:
@@ skipping 11 matching lines @@
         Profile* profile) const OVERRIDE;
     virtual bool ServiceIsCreatedWithProfile() OVERRIDE;
     virtual bool ServiceIsNULLWhileTesting() OVERRIDE;
   };
 
   ShellWindowSet shell_windows_;
   ObserverList<Observer> observers_;
 };
 
 #endif  // CHROME_BROWSER_EXTENSIONS_SHELL_WINDOW_REGISTRY_H_