Fixed coverity defects regarding pass-by-value

chrome/browser/extensions/shell_window_registry.cc

 // 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 "chrome/browser/extensions/shell_window_registry.h"
 #include "chrome/browser/profiles/profile_dependency_manager.h"
 #include "chrome/browser/ui/extensions/shell_window.h"
 #include "chrome/common/extensions/extension.h"
 #include "content/public/browser/render_view_host.h"
 #include "content/public/browser/web_contents.h"
@@ skipping 19 matching lines @@
 
 void ShellWindowRegistry::AddObserver(Observer* observer) {
   observers_.AddObserver(observer);
 }
 
 void ShellWindowRegistry::RemoveObserver(Observer* observer) {
   observers_.RemoveObserver(observer);
 }
 
 ShellWindowRegistry::ShellWindowSet ShellWindowRegistry::GetShellWindowsForApp(
-    const std::string app_id) const {
+    const std::string& app_id) const {
   ShellWindowSet app_windows;
   for (ShellWindowSet::const_iterator i = shell_windows_.begin();
        i != shell_windows_.end(); ++i) {
     if ((*i)->extension()->id() == app_id)
       app_windows.insert(*i);
   }
   return app_windows;
 }
 
 ShellWindow* ShellWindowRegistry::GetShellWindowForRenderViewHost(
@@ skipping 45 matching lines @@
   return new ShellWindowRegistry();
 }
 
 bool ShellWindowRegistry::Factory::ServiceIsCreatedWithProfile() {
   return true;
 }
 
 bool ShellWindowRegistry::Factory::ServiceIsNULLWhileTesting() {
   return false;
 }