StubCallInterfaceDescriptor and CallInterfaceDescriptor are unified under a base class InterfaceDescriptor.

Handling of the context register had to be massaged to effect the unification. This will make it easier to call hydrogen code stubs directly from crankshaft.

R=danno@chromium.org

Review URL: https://codereview.chromium.org/384403002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22448 ce2b1a6d-e550-0410-aec6-3dcde31c8c00