Introduce BeforeCallEnteredCallback.

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index b66057587f6d3250284986fda295154084db8e68..b263d5b0f700a0d3dbff1c9955a3e14486a7f3b1 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -1055,6 +1055,10 @@ 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);
@@ -1318,6 +1322,9 @@ 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_;