Extensions currently have their own URLRequestContext, which is used only for the cookie store.  This should be shifted over to being a CookieManager Mojo interface (probably implemented by a C++ wrapper around a CookieMonster).  Probably as part of this, it should be pulled out of the ProfileIOData that is initialized on the IO thread as part of profile initialization, and instead linked to a BrowserContextKeyedService at the Chrome level.  ExtensionSystemImpl would be an obvious choice.

Things this is related to:
* Converting uses of cookies in chrome/browser/extensions over to CookieManager
* Making renderer access to cookies go over the RestrictedCookieManager interface (because renderer processes which implement background pages for extensions should access cookies in the Extensions URLRequestContext).