Enable coalescing stream frames in QuicStreamFactoryFuzzer.

Hope the fuzzer will be able to crash and give out helpful stack trace.

Change-Id: Ie9a42b465eb966c10d52e4e263ed331a227d3e0a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1900261Reviewed-by: Zhongyi Shi <zhongyi@chromium.org>
Commit-Queue: Renjie Tang <renjietang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712801}

net/quic/quic_stream_factory_fuzzer.cc

@@ -143,6 +143,7 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
           &env->clock, params);
 
   SetQuicReloadableFlag(quic_supports_tls_handshake, true);
+  SetQuicReloadableFlag(quic_coalesce_stream_frames, true);
   QuicStreamRequest request(factory.get());
   TestCompletionCallback callback;
   NetErrorDetails net_error_details;