libvpx

vp10: merge frame_parallel_decoding_mode and refresh_frame_context.

See issue 1030. The value of frame_parallel_decoding_mode was ignored
in vp9 if refresh_frame_context was 0, so instead make it a 3-member
enum where the dependency is obviously stated.

Change-Id: I37f0177e5759f54e2e6cc6217023d5681de92438