Make reset_frame_context an enum.

Make reset_frame_context an enum.

In vp9, [0] and [1] had identical meaning, so merge them into a
single value. Make it impossible to code RESET_FRAME_CONTEXT_NONE
for intra_only frames, since that is a non-sensical combination.

See issue 1030.

Change-Id: If450c74162d35ca63a9d279beaa53ff9cdd6612b