Remove HttpDecoder::set_visitor().

Since all call sites have a visitor ready when HttpDecoder is instantiated, pass
that in HttpDecoder constructor instead of a set_visitor() method.  Also make
HttpDecoder::visitor_ member constant.  Just one fewer way to hold HttpDecoder
wrong.

While touching this area already, use |QuicMakeUnique| instead of |new| for
initializing unique_ptr<HttpDecoderVisitor> members.  And add quic_ptr_util.h
include to every file touched that needs it, even if no QuicMakeUnique is added
with this CL.

Also remove unused QuicReceiveControlStreamTest::decoder_ member.

gfe-relnote: n/a, change only affects QUIC v99.
PiperOrigin-RevId: 257306155
Change-Id: I6b27704dc6001f69668a36cd6072aaa24a95b389