Support rollback with installerdata in mini_installer

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 1498 matching lines @@
                   "too many distribution types");
     const Products& products = installer_state.products();
     for (Products::const_iterator it = products.begin(); it < products.end();
          ++it) {
       const Product& product = **it;
       const ProductState* product_state =
           original_state.GetProductState(system_install,
                                          product.distribution()->GetType());
       if (product_state != NULL &&
           (product_state->version().CompareTo(*installer_version) > 0)) {
         LOG(ERROR) << "Higher version of "

[grt (UTC plus 2), 2016/04/28 16:55:30]

i think this ERROR will be confusing to see in the logs when downgrade is
allowed. could you make the log level conditional? VLOG(1) if allowed, ERROR
otherwise?

[zmin, 2016/04/28 17:51:29]

How about we also bypass this part when downgrade is allowed. If we don't care
about whether there's higher version chrome or not, it doesn't make sense to
find them as well.

[grt (UTC plus 2), 2016/04/28 20:13:45]

sgtm

                    << product.distribution()->GetDisplayName()
                    << " is already installed.";
         higher_products |= (1 << product.distribution()->GetType());
       }
     }
 
-    if (higher_products != 0) {
+    if (higher_products != 0 && !CanDowngradeVersion(prefs)) {
       static_assert(BrowserDistribution::NUM_TYPES == 3,
                     "add support for new products here");
       int message_id = IDS_INSTALL_HIGHER_VERSION_BASE;
       proceed_with_installation = false;
       install_status = HIGHER_VERSION_EXISTS;
       installer_state.WriteInstallerResult(install_status, message_id, NULL);
     }
 
     if (proceed_with_installation) {
       base::FilePath prefs_source_path(cmd_line.GetSwitchValueNative(
@@ skipping 311 matching lines @@
     // to pass through, since this is only returned on uninstall which is
     // never invoked directly by Google Update.
     return_code = InstallUtil::GetInstallReturnCode(install_status);
   }
 
   installer::EndPersistentHistogramStorage(installer_state.target_path());
   VLOG(1) << "Installation complete, returning: " << return_code;
 
   return return_code;
 }