misc-fix: use enum for reset_frame_context consts

This commit merge the fixes that address the issues with the syntax
element reset_frame_context, and the conflict between frame_parallel_
decoding_mode and refresh_frame_context.

Related tracking issue:
https://bugs.chromium.org/p/webm/issues/detail?id=1030
Original libvpx commit:
62da0bf162f369cd99f2ffe37e4cb21810422e37
d1474f02aaf324112e63879b12c0f61553eed2a8

Change-Id: Iff0369c272d5eca5207b307a70911965a1f2f404
2 files changed