[chaotic-good] Fix use-after free (#37019)

When starting a call, we could delete the transport before acquiring the party lock for the call handler, leading to a use-after-free.

Closes #37019

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37019 from ctiller:chaotic-bug 58254aa19c
PiperOrigin-RevId: 645542043

src/core/ext/transport/chaotic_good/client_transport.cc

@@ -316,14 +316,16 @@ auto ChaoticGoodClientTransport::CallOutboundLoop(uint32_t stream_id,
 void ChaoticGoodClientTransport::StartCall(CallHandler call_handler) {
   // At this point, the connection is set up.
   // Start sending data frames.
-  call_handler.SpawnGuarded("outbound_loop", [this, call_handler]() mutable {
-    const uint32_t stream_id = MakeStream(call_handler);
-    return Map(CallOutboundLoop(stream_id, call_handler),
-               [stream_id, sender = outgoing_frames_.MakeSender()](
+  call_handler.SpawnGuarded(
+      "outbound_loop", [self = RefAsSubclass<ChaoticGoodClientTransport>(),
+                        call_handler]() mutable {
+        const uint32_t stream_id = self->MakeStream(call_handler);
+        return Map(self->CallOutboundLoop(stream_id, call_handler),
+                   [stream_id, sender = self->outgoing_frames_.MakeSender()](
                        absl::Status result) mutable {
                      GRPC_TRACE_LOG(chaotic_good, INFO)
-                     << "CHAOTIC_GOOD: Call " << stream_id << " finished with "
-                     << result.ToString();
+                         << "CHAOTIC_GOOD: Call " << stream_id
+                         << " finished with " << result.ToString();
                      if (!result.ok()) {
                        GRPC_TRACE_LOG(chaotic_good, INFO)
                            << "CHAOTIC_GOOD: Send cancel";