sb_size, mib_size and mib_size_log2 have been moved from the AV1_COMMON struct
to the SequenceHeader struct, as they are sequence-level parameters. sb_size is
now written to and read from the end of the sequence header.

BUG=aomedia:996

Change-Id: I9d450518a1b898677976acd80088b8ea9ee9d7ce