Two WebView API cleanups

- remove final from classes which we will need to provide subclasses
  in future: CookieManager, GeolocationPermissions, WebIconDatabase
  and WebStorage. None of these have published constructors,
  so applications cannot subclass them anyway.

- Also convert some protected members of JsResult to private, as its of
  no use to legal subclasses, and applications cannot subclass it.

Change-Id: Iaca9d2db31e25853b6c55feae41d9e7774087479