Introduce BeforeCallEnteredCallback.

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index d8ad32f742202fa191ff65e6675aa74eac5f4ceb..68d3552cb47d99db901c49c928551e0c19021148 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -1053,6 +1053,10 @@ class Isolate {
   void RemoveCallCompletedCallback(CallCompletedCallback callback);
   void FireCallCompletedCallback();
 
+  void AddBeforeCallEnteredCallback(BeforeCallEnteredCallback callback);
+  void RemoveBeforeCallEnteredCallback(BeforeCallEnteredCallback callback);
+  void FireBeforeCallEnteredCallback();
+
   void SetPromiseRejectCallback(PromiseRejectCallback callback);
   void ReportPromiseReject(Handle<JSObject> promise, Handle<Object> value,
                            v8::PromiseRejectEvent event);
@@ -1315,6 +1319,9 @@ class Isolate {
   int next_unique_sfi_id_;
 #endif
 
+  // List of callbacks before a Call starts execution.
+  List<BeforeCallEnteredCallback> before_call_entered_callbacks_;
+
   // List of callbacks when a Call completes.
   List<CallCompletedCallback> call_completed_callbacks_;