Fix comment for aom_codec_stream_info_t parameter.
Fix comment for aom_codec_stream_info_t parameter.

The size field of aom_codec_stream_info_t was removed in commit
afe71d9d520c55e328791b02c49ab44171299153. Also, the 'si' parameter
cannot be NULL.

Change-Id: Ie4b3fa3c570939c4330f6111fbc8838da4f9036d

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