Make application shortcuts point to app_host.exe, install App Host during app installation.

chrome/browser/extensions/app_host_installer.h

Index: chrome/browser/extensions/app_host_installer.h
diff --git a/chrome/browser/extensions/app_host_installer.h b/chrome/browser/extensions/app_host_installer.h
new file mode 100644
index 0000000000000000000000000000000000000000..e434a388f5aa298cc09b2f729dc7c4d4e73d91b8
--- /dev/null
+++ b/chrome/browser/extensions/app_host_installer.h
@@ -0,0 +1,34 @@
+// 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_INSTALLER_H_
+#define CHROME_BROWSER_EXTENSIONS_APP_HOST_INSTALLER_H_
+
+#include "base/basictypes.h"
+#include "base/callback_forward.h"
+
+namespace extensions {
+
+class Extension;
+
+class AppHostInstaller {
+ public:
+  AppHostInstaller();
+
+  // Installs the App Host if it is required and not present for the given
+  // extension on the current platform.
+  // Calls |completion_callback| after checks and installation (if necessary)
+  // are complete, with a boolean which will be false iff the installation was
+  // required but failed.
+  void AppHostInstaller::InstallAppHostIfNecessary(

[benwells, 2012/10/08 06:14:33]

Can this just be a static function in a namespace?

If not, lose the AppHostInstaller:: from the declaration.

[huangs, 2012/10/09 18:25:37]

Done.

+      const Extension& extension,
+      const base::Callback<void(bool)>& completion_callback);

[benwells, 2012/10/08 06:14:33]

I know I suggested adding the extension here, but it feels a bit wrong somehow.
If it wasn't here, this file and the implementation could move somewhere else,
which would make me happy. It does not feel right in extensions.

[erikwright (departed), 2012/10/09 15:54:53]

We can push the is_platform_app() check to call sites, but that doesn't seem
right either.

Currently the app_host_installer is in two parts - the basic outside part and
the platform-specific part.

The check for is_platform_app() could be kept in the outside part, and that
could stay here as a static helper function.

The platform-specific part could then be moved somewhere else.

I'd like to defer the actual question of where, while I think about it some
more.

[huangs, 2012/10/09 18:25:37]

Deferring.  Once we have a decision, will move the file as part of the final
change, to not disrupt diffs.

[benwells, 2012/10/10 03:22:04]

My main concerns are
(1) keeping all this code out of the extensions area, as it has little relation
to the other code there. One sign of this is that it should be reviewed by
installer folk, not extension folk.
(2) keep the flow of extension install simple for non-app-host-requiring
situations. This will help the extension folk who have to debug problems with
extension install and don't know anything about the app host.

Erik's suggestion has improved (2). 

Or, add something to extension.cc along the lines of:

bool Extension::requires_app_host() {
  return is_platform_app();
}

+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(AppHostInstaller);
+};
+
+}  // namespace extensions
+
+#endif  // CHROME_BROWSER_EXTENSIONS_APP_HOST_INSTALLER_H_