Do not call HttpDecoder::ProcessInput after error.

This CL changes API contract and call sites.  Without the change to the call
sites, //gfe/gfe2/test_tools/fuzzing:gfe_quic_fuzzer would crash on the new
DCHECK_NE(QUIC_NO_ERROR, error_).  This is because even if Visitor::OnError()
closes the connection, the sequencer might have more data that QuicSpdyStream is
eager to read and feed to HttpDecoder::ProcessInput().

Currently HttpDecoder::ProcessInput() ignores input upon error.  However, I'm
going to modify the call sites to use PeekRegion() instead of
PrefetchNextRegion() and spin until decoding is paused by a visitor method or no
new region is there to peek at.  Since HttpDecoder::ProcessInput() returns 0
if called after an error, this would just result in PeekRegion() being called
with the same offset, returning true, and the cycle spinning in an infinite
loop.

gfe-relnote: n/a, change to QUIC v99-only code path.
PiperOrigin-RevId: 255168809
Change-Id: I6b6f0b0986f2f6bec0a8eedb695dc91943d57bf2