Add DCHECK to v99 code path in QuicSpdyStream.

We really should not feed any data to HttpDecoder if sequencer is closed, that
is, at least |close_offset_| bytes have been consumed.  See
https://cs.corp.google.com/quiche/quic/core/quic_stream_sequencer.cc?l=199.

Remove QuicSpdyStreamTest.ReceivingTrailersOnRequestStream test which violates
this assumption because it feeds trailers into the stream before data.  If order
is fixed, this test becomes redundant with
QuicSpdyStreamTest.ProcessBodyAfterTrailers, which this CL improves a bit.

This came up during my investigation of https://crbug.com/969391, where the
input data found by the fuzzer would trip over this new DCHECK.

PiperOrigin-RevId: 253916860
Change-Id: I489710f132c7b6d1b00a203d871701bc2b52685c