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

net/spdy/header_coalescer.h

+// 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 {
+ 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_