Renaming skip_coeff to skip for consistency.

vp9/common/vp9_blockd.h

vp9/common/vp9_debugmodes.c

vp9/common/vp9_loopfilter.c

vp9/common/vp9_postproc.c

vp9/common/vp9_pred_common.c

vp9/common/vp9_pred_common.h

vp9/decoder/vp9_decodeframe.c

vp9/decoder/vp9_decodemv.c

vp9/encoder/vp9_bitstream.c

vp9/encoder/vp9_encodeframe.c

vp9/encoder/vp9_encodemb.c

vp9/encoder/vp9_encodemb.h

vp9/encoder/vp9_pickmode.c

vp9/encoder/vp9_rdopt.c

vp9/encoder/vp9_tokenize.c