[debugger] simplify debug stepping.

src/isolate.cc

Index: src/isolate.cc
diff --git a/src/isolate.cc b/src/isolate.cc
index a2500c0a6d9e16dfb10ad75a866aea135854b7f3..3a78cfc519fe65f6dc7aa58282c3b66e7e63529a 100644
--- a/src/isolate.cc
+++ b/src/isolate.cc
@@ -2781,16 +2781,19 @@ void Isolate::RemoveCallCompletedCallback(CallCompletedCallback callback) {
 
 
 void Isolate::FireCallCompletedCallback() {
-  bool has_call_completed_callbacks = !call_completed_callbacks_.is_empty();
+  if (!handle_scope_implementer()->CallDepthIsZero()) return;
+
   bool run_microtasks =
       pending_microtask_count() &&
       !handle_scope_implementer()->HasMicrotasksSuppressions() &&
       handle_scope_implementer()->microtasks_policy() ==
           v8::MicrotasksPolicy::kAuto;
-  if (!has_call_completed_callbacks && !run_microtasks) return;
 
-  if (!handle_scope_implementer()->CallDepthIsZero()) return;
   if (run_microtasks) RunMicrotasks();
+  // Prevent stepping from spilling into the next call made by the embedder.
+  if (debug()->is_active()) debug()->ClearStepping();
+
+  if (call_completed_callbacks_.is_empty()) return;
   // Fire callbacks.  Increase call depth to prevent recursive callbacks.
   v8::Isolate* isolate = reinterpret_cast<v8::Isolate*>(this);
   v8::Isolate::SuppressMicrotaskExecutionScope suppress(isolate);