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:

   * Updated spec
   * Changed Huffman code length alphabet to use run length codes more
     efficiently, based on a suggestion by Robert Obryk
   * Changed encoding of the number of Huffman code lengths (HLEN)
   * Changed encoding of the number of Huffman trees (NTREES)
   * Added support for uncompressed meta-blocks

brotli/enc/histogram.h

@@ -78,6 +78,10 @@ typedef Histogram<256> HistogramLiteral;
 typedef Histogram<kNumCommandPrefixes> HistogramCommand;
 typedef Histogram<kNumDistancePrefixes> HistogramDistance;
 typedef Histogram<kNumBlockLenPrefixes> HistogramBlockLength;
+// Context map histogram, 256 Huffman tree indexes + 16 run length codes.
+typedef Histogram<272> HistogramContextMap;
+// Block type histogram, 256 block types + 2 special symbols.
+typedef Histogram<258> HistogramBlockType;
 
 static const int kLiteralContextBits = 6;
 static const int kDistanceContextBits = 2;