If Chrome Binaries version > App Host version, then update App Host.

chrome/installer/launcher_support/chrome_launcher_support.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_INSTALLER_LAUNCHER_SUPPORT_CHROME_LAUNCHER_SUPPORT_H_
 #define CHROME_INSTALLER_LAUNCHER_SUPPORT_CHROME_LAUNCHER_SUPPORT_H_
+#include "base/string16.h"
 
 class FilePath;
 
 namespace chrome_launcher_support {
 
 enum InstallationLevel {
   USER_LEVEL_INSTALLATION,
   SYSTEM_LEVEL_INSTALLATION
 };
 
 // Returns the path to an installed chrome.exe, or an empty path. Prefers a
 // system-level installation to a user-level installation. Uses Omaha client
 // state to identify a Chrome installation location.
 // In non-official builds, to ease development, this will first look for a
 // chrome.exe in the same directory as the current executable.
 // The file path returned (if any) is guaranteed to exist.
 FilePath GetAnyChromePath();
 
+// Returns the path to an existing setup.exe at the specified level, if it can
+// be found via Omaha client state.
+FilePath GetSetupExeForInstallationLevel(InstallationLevel level);
+
 // Returns the path to an installed chrome.exe at the specified level, if it can
 // be found via Omaha client state.
 FilePath GetChromePathForInstallationLevel(InstallationLevel level);
 
+// TODO: Comments

[erikwright (departed), 2012/09/12 15:53:50]

These new methods should be moved out of launcher_support into app_host itself
for now. Create new files (c/b/e/ah/app_host_upgrade.{h,cc}) for them and add
them to the app_host target.

[huangs, 2012/09/12 19:24:27]

Done.

+bool GetAppHostVersionString(string16* version_str);
+
+// TODO: Comments
+bool GetChromeBinariesVersionStringForInstallationLevel(InstallationLevel level,
+                                                        string16* version_str);
+
+// TODO: Comments
+bool GetAnyChromeBinariesVersionString(string16* version_str);
+
+// TODO: COmments.
+bool UpgradeAppHost();
+
 }  // namespace chrome_launcher_support
 
 #endif  // CHROME_INSTALLER_LAUNCHER_SUPPORT_CHROME_LAUNCHER_SUPPORT_H_