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 three weeks. Most important changes:

  * Added UTF8 context model for good text compression.
  * Simplified context modeling by having only 4 context modes.
  * Per-block context mode selection.
  * Faster backward copying and bit reading functions.
  * More efficient histogram coding.
  * Streaming support for the decoder and encoder.

brotli/enc/literal_cost.h

@@ -22,9 +22,11 @@
 
 namespace brotli {
 
-// Input: length of data, and the bytes.
-// Output: estimate of how many bits the literal will take entropy coded.
-void EstimateBitCostsForLiterals(size_t len, const uint8_t *data, float *cost);
+// Estimates how many bits the literals in the interval [pos, pos + len) in the
+// ringbuffer (data, mask) will take entropy coded and writes these estimates
+// to the ringbuffer (cost, mask).
+void EstimateBitCostsForLiterals(size_t pos, size_t len, size_t mask,
+                                 const uint8_t *data, float *cost);
 
 }  // namespace brotli