Force migrate all clients from multi-install back to single-install.

chrome/installer/setup/setup_main.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/installer/setup/setup_main.h"
 
 #include <windows.h>
 #include <msi.h>
 #include <shellapi.h>
 #include <shlobj.h>
@@ skipping 1687 matching lines @@
   bool system_install = false;
   prefs.GetBool(installer::master_preferences::kSystemLevel, &system_install);
   VLOG(1) << "system install is " << system_install;
 
   InstallationState original_state;
   original_state.Initialize();
 
   InstallerState installer_state;
   installer_state.Initialize(cmd_line, prefs, original_state);
 
+  VLOG(1) << "is_migrating_to_single is "
+          << installer_state.is_migrating_to_single();
+
   persistent_histogram_storage.set_storage_dir(
       installer::PersistentHistogramStorage::GetReportedStorageDir(
           installer_state.target_path()));
 
   installer::ConfigureCrashReporting(installer_state);
   installer::SetInitialCrashKeys(installer_state);
   installer::SetCrashKeysFromCommandLine(cmd_line);
 
   // Make sure the process exits cleanly on unexpected errors.
   base::EnableTerminationOnHeapCorruption();
@@ skipping 128 matching lines @@
     // Note that we allow the status installer::UNINSTALL_REQUIRES_REBOOT
     // to pass through, since this is only returned on uninstall which is
     // never invoked directly by Google Update.
     return_code = InstallUtil::GetInstallReturnCode(install_status);
   }
 
   VLOG(1) << "Installation complete, returning: " << return_code;
 
   return return_code;
 }