Add a SpdyFramer extension API implementation.

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 36 matching lines @@
   // GetWriteableBuffer() above.
   bool Seek(size_t length);
 
   // Populates this frame with a HTTP2 frame prefix using length information
   // from |capacity_|. The given type must be a control frame type.
   bool BeginNewFrame(const SpdyFramer& framer,
                      SpdyFrameType type,
                      uint8_t flags,
                      SpdyStreamId stream_id);
 
-  // Populates this frame with a HTTP2 frame prefix with length information.
-  // The given type must be a control frame type.
+  // Populates this frame with a HTTP2 frame prefix with type and length
+  // information.
   bool BeginNewFrame(const SpdyFramer& framer,
-                     SpdyFrameType type,
+                     uint8_t type,

[xunjieli, 2017/04/04 17:07:36]

minor nit: could you s/type/raw_frame_type to match the one in the cc file?
"raw_frame_type" is slightly more informative. 
And add a comment in the doc mentioning that we use raw uint8_t here because we
accept extension frame types that aren't explicitly defined in the SpdyFrameType
enum.

[Bence, 2017/04/04 18:46:38]

Done.

                      uint8_t flags,
                      SpdyStreamId stream_id,
                      size_t length);
 
   // Takes the buffer from the SpdyFrameBuilder.
   SpdySerializedFrame take() {
     SPDY_BUG_IF(output_ != nullptr) << "ZeroCopyOutputBuffer is used to build "
                                     << "frames. take() shouldn't be called";
     SPDY_BUG_IF(kMaxFrameSizeLimit < length_)
         << "Frame length " << length_
@@ skipping 73 matching lines @@
   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_