Document last_active_segid and preskip_segid.
Document last_active_segid and preskip_segid.

Change the type of preskip_segid to uint8_t to suggest it is a boolean.

Change-Id: I01d4e34780cd1d73397731291a78d762862bd599

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