Removes C++20-style designated initializers from nghttp2_test_utils.h.

PiperOrigin-RevId: 442909396

quiche/http2/adapter/nghttp2_test_utils.h

diff --git a/quiche/http2/adapter/nghttp2_test_utils.h b/quiche/http2/adapter/nghttp2_test_utils.h
index 00a2f4c..9b359f4 100644
--- a/quiche/http2/adapter/nghttp2_test_utils.h
+++ b/quiche/http2/adapter/nghttp2_test_utils.h
@@ -31,11 +31,13 @@
   }
 
   nghttp2_data_provider MakeDataProvider() {
+    nghttp2_data_source s;
+    s.ptr = this;
     return nghttp2_data_provider{
-        .source = {.ptr = this},
-        .read_callback = [](nghttp2_session*, int32_t, uint8_t*, size_t length,
-                            uint32_t* data_flags, nghttp2_data_source* source,
-                            void*) -> ssize_t {
+        s,
+        [](nghttp2_session*, int32_t, uint8_t*, size_t length,
+           uint32_t* data_flags, nghttp2_data_source* source,
+           void*) -> ssize_t {
           *data_flags |= NGHTTP2_DATA_FLAG_NO_COPY;
           auto* s = static_cast<TestDataSource*>(source->ptr);
           if (!s->is_data_available()) {