When I make a debug build I get a check failure here.  The code in question is in `async_fetch.cc:set_request_headers`:

```
DCHECK(!owns_request_headers_);
if (owns_request_headers_) {
  delete request_headers_;
}
request_headers_ = headers;
owns_request_headers_ = false;
```

(`SetRequestHeadersTakingOwnership` first calls `set_request_headers` and then sets `owns_request_headers_ = true`.)

It looks like `ctx->base_fetch` already has request headers.