Updates CallbackVisitor to accept nghttp2_session_callbacks by const reference.

This is a small quality of life improvement to make integration a bit easier.

PiperOrigin-RevId: 377992441
diff --git a/http2/adapter/callback_visitor_test.cc b/http2/adapter/callback_visitor_test.cc
index 4dfcb5f..695ad7f 100644
--- a/http2/adapter/callback_visitor_test.cc
+++ b/http2/adapter/callback_visitor_test.cc
@@ -27,7 +27,7 @@
 TEST(ClientCallbackVisitorUnitTest, ConnectionFrames) {
   testing::StrictMock<MockNghttp2Callbacks> callbacks;
   CallbackVisitor visitor(Perspective::kClient,
-                          MockNghttp2Callbacks::GetCallbacks(), &callbacks);
+                          *MockNghttp2Callbacks::GetCallbacks(), &callbacks);
 
   testing::InSequence seq;
 
@@ -73,7 +73,7 @@
 TEST(ClientCallbackVisitorUnitTest, StreamFrames) {
   testing::StrictMock<MockNghttp2Callbacks> callbacks;
   CallbackVisitor visitor(Perspective::kClient,
-                          MockNghttp2Callbacks::GetCallbacks(), &callbacks);
+                          *MockNghttp2Callbacks::GetCallbacks(), &callbacks);
 
   testing::InSequence seq;
 
@@ -158,7 +158,7 @@
 TEST(ServerCallbackVisitorUnitTest, ConnectionFrames) {
   testing::StrictMock<MockNghttp2Callbacks> callbacks;
   CallbackVisitor visitor(Perspective::kServer,
-                          MockNghttp2Callbacks::GetCallbacks(), &callbacks);
+                          *MockNghttp2Callbacks::GetCallbacks(), &callbacks);
 
   testing::InSequence seq;
 
@@ -196,7 +196,7 @@
 TEST(ServerCallbackVisitorUnitTest, StreamFrames) {
   testing::StrictMock<MockNghttp2Callbacks> callbacks;
   CallbackVisitor visitor(Perspective::kServer,
-                          MockNghttp2Callbacks::GetCallbacks(), &callbacks);
+                          *MockNghttp2Callbacks::GetCallbacks(), &callbacks);
 
   testing::InSequence seq;