Existing CefBrowserContext functionality is now split between
CefBrowserContext and AlloyBrowserContext. Runtime implementations of
CefBrowserContext will provide access to the content::BrowserContext and
Profile types via different inheritance paths. For example, the Alloy
runtime uses ChromeProfileAlloy and the Chrome runtime uses ProfileImpl.
This change also renames CefResourceContext to CefIOThreadState to more
accurately represent its purpose as it no longer needs to extend
content::ResourceContext.
