move MIN/MAX_HISTO_BITS to format_constants.h

(under the name MAX_HUFFMAN_BITS, since the specs read:
	int huffman_bits = ReadBits(3) + 2;
)

Change-Id: Ifc66576bbd8e48518d3d78a3f515f851cf1883dc

src/webp/format_constants.h

@@ -40,6 +40,9 @@
 #define NUM_DISTANCE_CODES           40
 #define CODE_LENGTH_CODES            19
 
+#define MIN_HUFFMAN_BITS             2  // min number of Huffman bits
+#define MAX_HUFFMAN_BITS             9  // max number of Huffman bits
+
 #define TRANSFORM_PRESENT            1  // The bit to be written when next data
                                         // to be read is a transform.
 #define NUM_TRANSFORMS               4  // Maximum number of allowed transform