Tracking bug for the standalone app_shell target for running certain platform apps without running chrome.

This requires moving code necessary for apps out from //chrome into //extensions, which also clarifies the extensions system a bit and enables the eventual removal of apps support from Chrome on non-Chrome OS.

Inspired by content_shell, app_shell includes just enough to run a Chrome app. Much of that functionality is still missing from app_shell.

The original goals ( issue 288226 ) still apply:
- minimal performance cost (less memory, less I/O, less CPU)
- minimal dependencies (no extraneous services, no chrome/browser/*)
- usable as a base for building a minimal standalone app
- easier/faster/reliable testing of extension runtime and app APIs

but we also intend to expand on the "minimal standalone app" goal by adding the capability to run multiple apps simultaneously, and to support extensions as well.

This is a meta-bug for making AppShell production-ready on Chrome OS, along with minimal Linux support. Since the project was suspended for some time, part of this involves simplify figuring out what works and what needs fixing.