Use an infobar instead of alert box for extension install error.

chrome/browser/extensions/extension_install_ui.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_EXTENSION_INSTALL_UI_H_
 #define CHROME_BROWSER_EXTENSIONS_EXTENSION_INSTALL_UI_H_
 #pragma once
 
 #include <string>
 
 #include "base/compiler_specific.h"
 #include "base/string16.h"
+#include "chrome/browser/extensions/crx_installer_error.h"
 
 class Browser;
 class SkBitmap;
 
 namespace extensions {
 class Extension;
 class ExtensionWebstorePrivateApiTest;
 }  // namespace extensions
 
 // Interface that should be implemented for each platform to display all the UI
 // around extension installation.
 class ExtensionInstallUI {
  public:
   static ExtensionInstallUI* Create(Browser* browser);
 
   virtual ~ExtensionInstallUI();
 
   // Called when an extension was installed.
   virtual void OnInstallSuccess(const extensions::Extension* extension,
                                 SkBitmap* icon) = 0;
   // Called when an extension failed to install.
-  virtual void OnInstallFailure(const string16& error) = 0;
+  virtual void OnInstallFailure(const CrxInstallerError& error) = 0;
 
   // Whether or not to show the default UI after completing the installation.
   virtual void SetSkipPostInstallUI(bool skip_ui) = 0;
 
   // TODO(asargent) Normally we navigate to the new tab page when an app is
   // installed, but we're experimenting with instead showing a bubble when
   // an app is installed which points to the new tab button. This may become
   // the default behavior in the future.
   virtual void SetUseAppInstalledBubble(bool use_bubble) = 0;
 
   // Opens apps UI and animates the app icon for the app with id |app_id|.
   static void OpenAppInstalledUI(Browser* browser, const std::string& app_id);
 
   // Disables showing UI (ErrorBox, etc.) for install failures. To be used only
   // in tests.
   static void DisableFailureUIForTests();
 
  protected:
   ExtensionInstallUI();
 };
 
 #endif  // CHROME_BROWSER_EXTENSIONS_EXTENSION_INSTALL_UI_H_