[debug] implement intuitive semantics for stepping over await call.

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index 5c724bf3ee8d48f6fde9fcc0f194df4d564e469c..ba21937edb2ee2a5cb178adfee70a67bfae6e8cc 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -4820,6 +4820,8 @@ void Heap::IterateStrongRoots(ObjectVisitor* v, VisitMode mode) {
   v->Synchronize(VisitorSynchronization::kTop);
   Relocatable::Iterate(isolate_, v);
   v->Synchronize(VisitorSynchronization::kRelocatable);
+  isolate_->debug()->Iterate(v);
+  v->Synchronize(VisitorSynchronization::kDebug);
 
   isolate_->compilation_cache()->Iterate(v);
   v->Synchronize(VisitorSynchronization::kCompilationCache);