Change tier field of SequenceHeader to uint8_t's.
Change tier field of SequenceHeader to uint8_t's.

Each tier[i] comes from one bit in the bitstream, so this field can be
an array of uint8_t's.

Change-Id: I123816282a978fdb9a793d8709bb4200b653a006

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom