Add HpackDecoder3, an adapter using HpackDecoder (in net/http2/hpack/decoder).

net/spdy/spdy_flags.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/spdy/spdy_flags.h"
 
 namespace net {
 
 // Log compressed size of HTTP/2 requests.
 bool FLAGS_chromium_http2_flag_log_compressed_size = true;
 
 // If true, remove use of SpdyFrameBuilder::OverwriteLength().
 bool FLAGS_chromium_http2_flag_remove_rewritelength = true;
 
-// Use //net/http2/hpack/decoder as HPACK decoder.
+// Use //net/http2/hpack/decoder as HPACK entry decoder.
 bool FLAGS_chromium_http2_flag_spdy_use_hpack_decoder2 = false;
 
+// Use //net/http2/hpack/decoder as complete HPACK decoder.
+bool FLAGS_chromium_http2_flag_spdy_use_hpack_decoder3 = false;
+
 // If true, increase HPACK table size up to optimal size kOptTableSize if
 // clients allow it.
 bool FLAGS_chromium_reloadable_flag_increase_hpack_table_size = false;
 
 // Use Http2FrameDecoderAdapter.
 bool FLAGS_use_http2_frame_decoder_adapter = false;
 
 // Use NestedSpdyFramerDecoder.
 bool FLAGS_use_nested_spdy_framer_decoder = false;
 
 }  // namespace net