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

chrome/browser/extensions/app_host/update.h

Index: chrome/browser/extensions/app_host/update.h
diff --git a/chrome/browser/extensions/app_host/update.h b/chrome/browser/extensions/app_host/update.h
new file mode 100644
index 0000000000000000000000000000000000000000..b80b766272e8d2b549637033e226eb6567e5f3bc
--- /dev/null
+++ b/chrome/browser/extensions/app_host/update.h
@@ -0,0 +1,16 @@
+// 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_APP_HOST_UPDATE_H_
+#define CHROME_BROWSER_EXTENSIONS_APP_HOST_UPDATE_H_
+
+namespace app_host {
+
+// If system-level Chrome Binary is installed, and if its version is
+// newer than App Host's, then udpate App Host by calling setup.exe.
+void EnsureAppHostUpToDate();
+
+}  // namespace app_host
+
+#endif  // CHROME_BROWSER_EXTENSIONS_APP_HOST_UPDATE_H_