Install CRX updates one at a time.

chrome/browser/extensions/crx_installer.h

 // Copyright (c) 2011 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_CRX_INSTALLER_H_
 #define CHROME_BROWSER_EXTENSIONS_CRX_INSTALLER_H_
 #pragma once
 
 #include <string>
 
@@ skipping 132 matching lines @@
 
   extension_misc::CrxInstallCause install_cause() const {
     return install_cause_;
   }
 
   void set_install_cause(extension_misc::CrxInstallCause install_cause) {
     install_cause_ = install_cause;
   }
 
  private:
+  friend class ExtensionUpdaterTest;
+
   virtual ~CrxInstaller();
 
   // Converts the source user script to an extension.
   void ConvertUserScriptOnFileThread();
 
   // Converts the source web app to an extension.
   void ConvertWebAppOnFileThread(const WebApplicationInfo& web_app);
 
   // Called after OnUnpackSuccess as a last check to see whether the install
   // should complete.
@@ skipping 114 matching lines @@
   std::string original_mime_type_;
 
   // What caused this install?  Used only for histograms that report
   // on failure rates, broken down by the cause of the install.
   extension_misc::CrxInstallCause install_cause_;
 
   DISALLOW_COPY_AND_ASSIGN(CrxInstaller);
 };
 
 #endif  // CHROME_BROWSER_EXTENSIONS_CRX_INSTALLER_H_