Avoid SpdyHeaderBlock copy in SpdyStream::OnPushPromiseHeadersReceived().

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_; }
+  SpdyHeaderBlock release_headers();
   bool error_seen() const { return error_seen_; }
 
  private:
   SpdyHeaderBlock headers_;
+  bool headers_valid_ = true;
   size_t header_list_size_ = 0;
   bool error_seen_ = false;
   bool regular_header_seen_ = false;
 };
 
 }  // namespace net
 
 #endif  // NET_SPDY_HEADER_COALESCER_H_