Allow stepping into Object.observe handlers.

src/runtime/runtime-debug.cc

Index: src/runtime/runtime-debug.cc
diff --git a/src/runtime/runtime-debug.cc b/src/runtime/runtime-debug.cc
index 68d2f9e4777d0ef30b9e558345ef37039eca26cc..7ab152cee3f3b8a8d8bc6ed83ab78502cde83263 100644
--- a/src/runtime/runtime-debug.cc
+++ b/src/runtime/runtime-debug.cc
@@ -2732,7 +2732,9 @@ RUNTIME_FUNCTION(Runtime_GetScript) {
 // to a built-in function such as Array.forEach.
 RUNTIME_FUNCTION(Runtime_DebugCallbackSupportsStepping) {
   DCHECK(args.length() == 1);
-  if (!isolate->debug()->is_active() || !isolate->debug()->StepInActive()) {
+  Debug* debug = isolate->debug();
+  if (!debug->is_active() || !debug->IsStepping() ||
+      debug->last_step_action() != StepIn) {
     return isolate->heap()->false_value();
   }
   CONVERT_ARG_CHECKED(Object, callback, 0);