ffi-buffer-cookie-policy and Electron

[aadcg]

ffi-buffer-cookie-policy was modeled after WebKitGTK's API (see cookie-policy type) which may not be valid for all renderers. For instance, Electron doesn't provide the possibility to disable cookies.

Solutions include moving this functionality outside foreign-interface and handle it on a renderer basis, or warn when some values aren't supported depending on the renderer being used.