Introduce --log-external-callbacks flag apart from --log-timer-events.

src/vm-state-inl.h

Index: src/vm-state-inl.h
diff --git a/src/vm-state-inl.h b/src/vm-state-inl.h
index bc481605fbf774034b45cb7ad4015b86e6ac1652..baa4b44932633f994d323d54db9c675eaa46d8ff 100644
--- a/src/vm-state-inl.h
+++ b/src/vm-state-inl.h
@@ -67,7 +67,7 @@ VMState::VMState(Isolate* isolate, StateTag tag)
     LOG(isolate, UncheckedStringEvent("From", StateToString(previous_tag_)));
   }
 
-  if (FLAG_log_timer_events) {
+  if (FLAG_log_external_callbacks) {
     LOG(isolate, ExternalSwitch(previous_tag_, tag));
   }
 
@@ -84,7 +84,7 @@ VMState::~VMState() {
         UncheckedStringEvent("To", StateToString(previous_tag_)));
   }
 
-  if (FLAG_log_timer_events) {
+  if (FLAG_log_external_callbacks) {
     LOG(isolate_, ExternalSwitch(isolate_->current_vm_state(), previous_tag_));
   }