Update zlib to 1.2.11

net/websockets/websocket_deflater.cc

Index: net/websockets/websocket_deflater.cc
diff --git a/net/websockets/websocket_deflater.cc b/net/websockets/websocket_deflater.cc
index 9144c0116dc1ab7a13084c45c2ad5534069df4ae..9e577ec33c34644e1b4d6f7b41c8d57a052f6838 100644
--- a/net/websockets/websocket_deflater.cc
+++ b/net/websockets/websocket_deflater.cc
@@ -31,11 +31,33 @@ bool WebSocketDeflater::Initialize(int window_bits) {
 
   DCHECK_LE(8, window_bits);
   DCHECK_GE(15, window_bits);
+
+  // Use a negative value to compress a raw deflate stream.
+  //
+  // Upgrade window_bits = 8 to 9 because zlib is unable to compress at
+  // window_bits = 8. Historically, zlib has silently increased the window size
+  // during compression in this case, although this is no longer done for raw
+  // deflate streams since zlib 1.2.9.
+  //
+  // Because of a zlib deflate quirk, back-references will not use the entire
+  // range of 1 << window_bits, but will instead use a restricted range of (1 <<
+  // window_bits) - 262. With an increased window_bits = 9, back-references will
+  // be within a range of 250. These can still be decompressed with window_bits
+  // = 8 and the 256-byte window used there.
+  //
+  // Both the requirement to do this upgrade and the ability to compress with
+  // window_bits = 9 while expecting a decompressor to function with window_bits
+  // = 8 are quite specific to zlib's particular deflate implementation, but not
+  // specific to any particular inflate implementation.
+  //
+  // See https://crbug.com/691074
+  window_bits = -std::max(window_bits, 9);
+
   memset(stream_.get(), 0, sizeof(*stream_));
   int result = deflateInit2(stream_.get(),
                             Z_DEFAULT_COMPRESSION,
                             Z_DEFLATED,
-                            -window_bits,  // Negative value for raw deflate
+                            window_bits,
                             8,  // default mem level
                             Z_DEFAULT_STRATEGY);
   if (result != Z_OK) {