net/http: inline chunkWriter in response
net/http: inline chunkWriter in response

A chunkWriter and a response are 1:1. Make them contiguous in
memory and save an allocation.

benchmark                                   old ns/op    new ns/op    delta
BenchmarkServerFakeConnWithKeepAliveLite        10715        10539   -1.64%

benchmark                                  old allocs   new allocs    delta
BenchmarkServerFakeConnWithKeepAliveLite           21           20   -4.76%

benchmark                                   old bytes    new bytes    delta
BenchmarkServerFakeConnWithKeepAliveLite         1626         1609   -1.05%

R=golang-dev, gri
CC=golang-dev
https://codereview.appspot.com/8114043

scirelli/scirelli-go,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,webfd/go-zh,scirelli/scirelli-go,bryanxu/go-zh,rdp/rogerpack2005-golang,mhennings/marcohennings-go,mhennings/marcohennings-go,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,mhennings/marcohennings-go,scirelli/scirelli-go,bryanxu/go-zh,webfd/go-zh,rdp/rogerpack2005-golang,scirelli/scirelli-go,sanjosh/sanjos100-tipc,webfd/go-zh,scirelli/scirelli-go,d0f/go-zh,bryanxu/go-zh,webfd/go-zh,d0f/go-zh,d0f/go-zh,scirelli/scirelli-go,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,rdp/rogerpack2005-golang,scirelli/scirelli-go,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,webfd/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,bryanxu/go-zh,mhennings/marcohennings-go,d0f/go-zh,webfd/go-zh,scirelli/scirelli-go,d0f/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,bryanxu/go-zh