Fix build errors in Chromium.

See https://chromium-review.googlesource.com/c/chromium/src/+/6083713

PiperOrigin-RevId: 704935268

quiche/web_transport/test_tools/in_memory_stream.h

diff --git a/quiche/web_transport/test_tools/in_memory_stream.h b/quiche/web_transport/test_tools/in_memory_stream.h
index 46e8d80..48da99a 100644
--- a/quiche/web_transport/test_tools/in_memory_stream.h
+++ b/quiche/web_transport/test_tools/in_memory_stream.h
@@ -37,6 +37,7 @@
   absl::Status Writev(absl::Span<const absl::string_view> data,
                       const quiche::StreamWriteOptions& options) override {
     QUICHE_NOTREACHED() << "Writev called on a read-only stream";
+    return absl::UnimplementedError("Writev called on a read-only stream");
   }
   bool CanWrite() const override { return false; }