Brotli format change: improved encoding of Huffman codes

This change removes the redundant HCLEN, HLENINC and HLEN
fields from the encoding of the complex Huffman codes and
derives these from an invariant of the code length sequence.
Based on a patch by Robert Obryk.

brotli/enc/bit_cost.h

@@ -104,7 +104,7 @@ static inline int HuffmanBitCost(const uint8_t* depth, int length) {
 template<int kSize>
 double PopulationCost(const Histogram<kSize>& histogram) {
   if (histogram.total_count_ == 0) {
-    return 11;
+    return 12;
   }
   int count = 0;
   for (int i = 0; i < kSize && count < 5; ++i) {
@@ -113,10 +113,10 @@ double PopulationCost(const Histogram<kSize>& histogram) {
     }
   }
   if (count == 1) {
-    return 11;
+    return 12;
   }
   if (count == 2) {
-    return 19 + histogram.total_count_;
+    return 20 + histogram.total_count_;
   }
   uint8_t depth[kSize] = { 0 };
   CreateHuffmanTree(&histogram.data_[0], kSize, 15, depth);
@@ -125,7 +125,9 @@ double PopulationCost(const Histogram<kSize>& histogram) {
     bits += histogram.data_[i] * depth[i];
   }
   if (count == 3) {
-    bits += 27;
+    bits += 28;
+  } else if (count == 4) {
+    bits += 37;
   } else {
     bits += HuffmanBitCost(depth, kSize);
   }