Enable LEB128 OBU size fields by default.
Enable LEB128 OBU size fields by default.

BUG=aomedia:1125

Change-Id: I78847969b708c258341a24e9b8aef228b28093aa

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