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

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 c5973ca5c2de45a254d03af6c85cf374343e8e56..fcd1d819976a8041d8d2bf00ada69e2e1d1327ba 100644
--- a/net/spdy/spdy_frame_builder.h
+++ b/net/spdy/spdy_frame_builder.h
@@ -138,6 +138,14 @@ class NET_EXPORT_PRIVATE SpdyFrameBuilder {
   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