Let the second pass phantom callbacks run in a separate task on the foreground thread.

test/cctest/cctest.h

Index: test/cctest/cctest.h
diff --git a/test/cctest/cctest.h b/test/cctest/cctest.h
index cc9edc801f5e268b17476178cea243b51cb2495b..ceb9f58157ad9bb471729ee302513c46988572bc 100644
--- a/test/cctest/cctest.h
+++ b/test/cctest/cctest.h
@@ -28,6 +28,7 @@
 #ifndef CCTEST_H_
 #define CCTEST_H_
 
+#include "include/libplatform/libplatform.h"
 #include "src/v8.h"
 
 #ifndef TEST
@@ -594,6 +595,13 @@ static inline void EnableDebugger() {
 static inline void DisableDebugger() { v8::Debug::SetDebugEventListener(NULL); }
 
 
+static inline void EmptyMessageQueues(v8::Isolate* isolate) {
+  while (v8::platform::PumpMessageLoop(v8::internal::V8::GetCurrentPlatform(),
+                                       isolate))
+    ;
+}
+
+
 // Helper class for new allocations tracking and checking.
 // To use checking of JS allocations tracking in a test,
 // just create an instance of this class.