Adds an OnFrameHeader() callback to Http2VisitorInterface.

PiperOrigin-RevId: 371390069
Change-Id: Icb2bb06ac19b42077e61c45c8d89ae7255030ee3
diff --git a/http2/adapter/nghttp2_callbacks.cc b/http2/adapter/nghttp2_callbacks.cc
index dc4962a..2568179 100644
--- a/http2/adapter/nghttp2_callbacks.cc
+++ b/http2/adapter/nghttp2_callbacks.cc
@@ -20,6 +20,8 @@
                  const nghttp2_frame_hd* header,
                  void* user_data) {
   auto* visitor = static_cast<Http2VisitorInterface*>(user_data);
+  visitor->OnFrameHeader(header->stream_id, header->length, header->type,
+                         header->flags);
   if (header->type == NGHTTP2_DATA) {
     visitor->OnBeginDataForStream(header->stream_id, header->length);
   }