Prepare SendChunkedPostRequestAbortedByResetStream for flag flip.

Use STOP_SENDING frame instead of RESET_FRAME and STOP_SENDING frames
together in SendChunkedPostRequestAbortedByResetStream to tell the
client not to transmit the rest of the request without signalling an
error.

This change is necessary for flipping quic_fix_on_stream_reset flag.

Change-Id: I8b2736a99ec93c850535304c0258063f4489472f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2716403
Reviewed-by: Renjie Tang <renjietang@chromium.org>
Commit-Queue: Bence Béky <bnc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#857873}