[promises] Refactor debug code

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index 36c22bbddd18f4342ed42dd3b4d11fccd4dab9f0..546228fceeedcae3df6c1ea6804960355a3f8f84 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -407,7 +407,7 @@ typedef List<HeapObject*> DebugObjectCache;
   V(v8::DeserializeInternalFieldsCallback,                                    \
     deserialize_internal_fields_callback, nullptr)                            \
   V(int, pending_microtask_count, 0)                                          \
-  V(int, debug_microtask_count, 0)                                            \
+  V(int, debug_microtask_count, DEBUG_PROMISE_DEFAULT_ID + 1)                 \
   V(HStatistics*, hstatistics, nullptr)                                       \
   V(CompilationStatistics*, turbo_statistics, nullptr)                        \
   V(HTracer*, htracer, nullptr)                                               \