Create simplified ExtensionSystem for app_shell

* Only initialize a few core ExtensionSystem services
* Switch to using extension_file_util::LoadExtension instead of AppLoadService,
  which allows us to avoid ExtensionService for now.
* Clean up RootWindow creation and handle clicks on the close box.

This allows app_shell to parse a non-app extension and create an Extension
object. The Extension does not yet start running.

BUG=288226
TEST=none
R=derat@chromium.org, yoz@chromium.org

Review URL: https://codereview.chromium.org/116573011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241880 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241880