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This change introduces separate ExtensionSystemFactories for app shell and for chrome; in particular, a new ShellExtensionSystemFactory. ExtensionsBrowserClient now selects between the two at the factory level. Callers can simply use ExtensionSystem::Get.

BUG=337707

Review URL: https://codereview.chromium.org/137753016

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251014 0039d316-1c4b-4281-b951-d872f2087c98