Add a SpdyFramer extension API implementation.

net/spdy/spdy_frame_builder.h

Index: net/spdy/spdy_frame_builder.h
diff --git a/net/spdy/spdy_frame_builder.h b/net/spdy/spdy_frame_builder.h
index 2715452d90b702b9c371fb32f5f2027562bca9de..9b187260cb161ea90a1d01338428f173ad980042 100644
--- a/net/spdy/spdy_frame_builder.h
+++ b/net/spdy/spdy_frame_builder.h
@@ -54,10 +54,10 @@ class NET_EXPORT_PRIVATE SpdyFrameBuilder {
                      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);