Create new versions of SerializeXXX() functions that uses pre-allocated buffer in SpdyFrameBuilder.

net/spdy/spdy_frame_builder.h

 // Copyright (c) 2012 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_SPDY_FRAME_BUILDER_H_
 #define NET_SPDY_SPDY_FRAME_BUILDER_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 120 matching lines @@
   // A buffer to be created whenever a new frame needs to be written. Used only
   // if |output_| is nullptr.
   std::unique_ptr<char[]> buffer_;
   // A pre-allocated buffer. If not-null, serialized frame data is written to
   // this buffer.
   ZeroCopyOutputBuffer* output_ = nullptr;  // Does not own.
 
   size_t capacity_;  // Allocation size of payload, set by constructor.
   size_t length_;    // Length of the latest frame in the buffer.
   size_t offset_;    // Position at which the latest frame begins.
+
+  // Remove all four below after
+  // FLAGS_chromium_http2_flag_remove_rewritelength deprecates.
+  const size_t kLengthFieldLength = 3;
+  char* start_of_current_frame_ = nullptr;
+  size_t bytes_of_length_written_in_first_block_ = kLengthFieldLength;
+  // In case length of a new frame is cross blocks.
+  char* start_of_current_frame_in_next_block_ = nullptr;
 };
 
 }  // namespace net
 
 #endif  // NET_SPDY_SPDY_FRAME_BUILDER_H_