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

net/spdy/spdy_flags.cc

Index: net/spdy/spdy_flags.cc
diff --git a/net/spdy/spdy_flags.cc b/net/spdy/spdy_flags.cc
index fa380d4921ccf1eb4506e59483876cf28d0afacc..1f342d8916a0497ee4d4d3776509a31f04984d96 100644
--- a/net/spdy/spdy_flags.cc
+++ b/net/spdy/spdy_flags.cc
@@ -12,9 +12,12 @@ 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;