Comment zcoeff_blk.

Change-Id: Iefc2eb78e71472ecf51802ec59ff32caef4bd0f4

vp9/encoder/vp9_block.h

@@ -100,7 +100,10 @@ struct macroblock {
   int mv_row_min;
   int mv_row_max;
 
+  // Notes transform blocks where no coefficents are coded.
+  // Set during mode selection. Read during block encoding.
   uint8_t zcoeff_blk[TX_SIZES][256];
+
   int skip;
 
   int encode_breakout;