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

net/spdy/header_coalescer.cc

Index: net/spdy/header_coalescer.cc
diff --git a/net/spdy/header_coalescer.cc b/net/spdy/header_coalescer.cc
new file mode 100644
index 0000000000000000000000000000000000000000..8e1bf5d184d6266a1d623d5e2aea4300c916dc2a
--- /dev/null
+++ b/net/spdy/header_coalescer.cc
@@ -0,0 +1,47 @@
+// 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.
+
+#include "net/spdy/header_coalescer.h"
+
+#include "base/strings/string_util.h"
+
+namespace net {
+
+void HeaderCoalescer::OnHeader(base::StringPiece key, base::StringPiece value) {
+  if (key.empty()) {
+    DVLOG(1) << "Header name must not be empty.";
+    error_seen_ = true;
+    return;
+  }
+
+  for (char c : key) {
+    if (base::IsAsciiUpper(c)) {
+      DLOG(ERROR) << "Malformed header: Header name " << key
+                  << " contains upper-case characters.";
+      error_seen_ = true;
+      return;
+    }
+  }
+
+  auto iter = headers_.find(key);
+  if (iter == headers_.end()) {
+    headers_[key] = value;
+  } else if (key == "cookie") {
+    // Obeys section 8.1.2.5 in RFC 7540 for cookie reconstruction.
+    base::StringPiece v = iter->second;
+    std::string s(v.data(), v.length());
+    s.append("; ");
+    value.AppendToString(&s);
+    headers_.ReplaceOrAppendHeader(key, s);
+  } else {
+    // This header had multiple values, so it must be reconstructed.
+    base::StringPiece v = iter->second;
+    std::string s(v.data(), v.length());
+    base::StringPiece("\0", 1).AppendToString(&s);
+    value.AppendToString(&s);
+    headers_.ReplaceOrAppendHeader(key, s);
+  }
+}
+
+}  // namespace net