Signal sb_size for intra-only frames.
Signal sb_size for intra-only frames.

BUG=aomedia:938

Change-Id: Ic730ac1136e915d193f3b48f7788baa093e7ff0f

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