Avoids the "re-encode HPACK as SPDY3" step.

net/spdy/header_coalescer.h

Index: net/spdy/header_coalescer.h
diff --git a/net/spdy/header_coalescer.h b/net/spdy/header_coalescer.h
new file mode 100644
index 0000000000000000000000000000000000000000..e699622517d09d70ae8c474de6f0c07e60bc29d4
--- /dev/null
+++ b/net/spdy/header_coalescer.h
@@ -0,0 +1,34 @@
+// Copyright (c) 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.
+
+#ifndef NET_SPDY_HEADER_COALESCER_H_
+#define NET_SPDY_HEADER_COALESCER_H_
+
+#include "net/base/net_export.h"
+#include "net/spdy/spdy_header_block.h"
+#include "net/spdy/spdy_headers_handler_interface.h"
+
+namespace net {
+
+class NET_EXPORT_PRIVATE HeaderCoalescer : public SpdyHeadersHandlerInterface {

[Ryan Hamilton, 2016/05/13 22:03:15]

Is this class from the internal repository? It looks very similar to
SpdyUtils::CopyAndValidateHeaders. I'm nervous about chrome using different code
to assemble headers than other classes. Can we share code instead?

[dahollings, 2016/05/16 22:00:17]

This class is not from the internal repository-- we created it as an analogue to
the internal spdy_headers_handler class. I see your point about the similarities
with SpdyUtils::CopyAndValidateHeaders, but I'm not sure how best to share the
code. The new changes need a class implementing the handler interface to pass to
hpack_decoder, as it now builds a HeaderBlock up by repeated calls to OnHeader.
CopyAndValidate receives a QuicHeaderList from the beginning. Suggestions will
be welcome!

[Ryan Hamilton, 2016/05/16 22:08:43]

*nod* And since that traffics in BalsaHeaders, it's be a poor fit here :(
Since we have Chrome, and the QUIC toy client and server parsing headers, it
would be good for them to all share the same implementation. I'm not sure that I
understand what you mean about the difference between the needs of
QuicChromiumClientStream (which in this CL uses the coalescer) and
QuicSpdyClientStream (which uses SpdyUtils::CopyAndValidateHeaders). I'm sure
*you* understand it, however :> Care to hop on VC and we can discuss at higher
bandwidth?

+ public:
+  HeaderCoalescer() {}
+
+  void OnHeaderBlockStart() override {}
+
+  void OnHeader(base::StringPiece key, base::StringPiece value) override;
+
+  void OnHeaderBlockEnd(size_t uncompressed_header_bytes) override {}
+
+  const SpdyHeaderBlock& headers() const { return headers_; }
+  bool error_seen() const { return error_seen_; }
+
+ private:
+  SpdyHeaderBlock headers_;
+  bool error_seen_ = false;
+};
+
+}  // namespace net
+
+#endif  // NET_SPDY_HEADER_COALESCER_H_