GitLab

* Create ExtensionSystem and register its dependencies from the ExtensionsBrowserClient interface.
* Create and tear down BrowserContextKeyedServices using the dependency manager like you're supposed to do it.
* Don't register AppLoaderServiceFactory - we don't use it anymore.
* Remove the "custom instance" code to ExtensionSystem::Get() - it was kind of hacky.

BUG=none
TEST=Launch app_shell then close the window. No crash.

Review URL: https://codereview.chromium.org/118043003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244013 0039d316-1c4b-4281-b951-d872f2087c98