Use grpc_core::RefCount for grpc_call and mark Unref path unlikely.

Unfortunately, we cannot use RefCount for `batch`, unless we support
reset API. So, for now, let's mark its unref path as unlikely.

src/core/lib/surface/call.cc

@@ -39,6 +39,7 @@
 #include "src/core/lib/gpr/useful.h"
 #include "src/core/lib/gprpp/arena.h"
 #include "src/core/lib/gprpp/manual_constructor.h"
+#include "src/core/lib/gprpp/ref_counted.h"
 #include "src/core/lib/iomgr/timer.h"
 #include "src/core/lib/profiling/timers.h"
 #include "src/core/lib/slice/slice_internal.h"
@@ -130,7 +131,6 @@ struct grpc_call {
         channel(args.channel),
         is_client(args.server_transport_data == nullptr),
         stream_op_payload(context) {
-    gpr_ref_init(&ext_ref, 1);
     for (int i = 0; i < 2; i++) {
       for (int j = 0; j < 2; j++) {
         metadata_batch[i][j].deadline = GRPC_MILLIS_INF_FUTURE;
@@ -142,7 +142,7 @@ struct grpc_call {
     gpr_free(static_cast<void*>(const_cast<char*>(final_info.error_string)));
   }
 
-  gpr_refcount ext_ref;
+  grpc_core::RefCount ext_ref;
   grpc_core::Arena* arena;
   grpc_core::CallCombiner call_combiner;
   grpc_completion_queue* cq;
@@ -553,10 +553,10 @@ static void destroy_call(void* call, grpc_error* error) {
                                             grpc_schedule_on_exec_ctx));
 }
 
-void grpc_call_ref(grpc_call* c) { gpr_ref(&c->ext_ref); }
+void grpc_call_ref(grpc_call* c) { c->ext_ref.Ref(); }
 
 void grpc_call_unref(grpc_call* c) {
-  if (!gpr_unref(&c->ext_ref)) return;
+  if (GPR_LIKELY(!c->ext_ref.Unref())) return;
 
   GPR_TIMER_SCOPE("grpc_call_unref", 0);
 
@@ -1225,7 +1225,7 @@ static void post_batch_completion(batch_control* bctl) {
 }
 
 static void finish_batch_step(batch_control* bctl) {
-  if (gpr_unref(&bctl->steps_to_complete)) {
+  if (GPR_UNLIKELY(gpr_unref(&bctl->steps_to_complete))) {
     post_batch_completion(bctl);
   }
 }