Updates to Brotli compression format, decoder and encoder

This commit contains a batch of changes that were made to the Brotli
compression algorithm in the last month. Most important changes:

   * Fixes to the spec.
   * Change of code length code order.
   * Use a 2-level Huffman lookup table in the decoder.
   * Faster uncompressed meta-block decoding.
   * Optimized encoding of the Huffman code.
   * Detection of UTF-8 input encoding.
   * UTF-8 based literal cost modeling for improved
     backward reference selection.

brotli/enc/entropy_encode.h

@@ -86,7 +86,7 @@ void BuildEntropyCode(const Histogram<kSize>& histogram,
       ++code->count_;
     }
   }
-  if (code->count_ >= 64) {
+  if (alphabet_size >= 50 && code->count_ >= 16) {
     int counts[kSize];
     memcpy(counts, &histogram.data_[0], sizeof(counts[0]) * kSize);
     OptimizeHuffmanCountsForRle(alphabet_size, counts);