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

test/cctest/test-api.cc

Index: test/cctest/test-api.cc
diff --git a/test/cctest/test-api.cc b/test/cctest/test-api.cc
index 1433b7bed6b86a6a1a664818e34bb47c786618de..234e8cee598197473e6f7abbbea73675a0b77111 100644
--- a/test/cctest/test-api.cc
+++ b/test/cctest/test-api.cc
@@ -36,6 +36,7 @@
 #include <unistd.h>  // NOLINT
 #endif
 
+#include "include/libplatform/libplatform.h"
 #include "include/v8-util.h"
 #include "src/api.h"
 #include "src/arguments.h"
@@ -88,6 +89,11 @@ using ::v8::Value;
   }                                                                  \
   THREADED_TEST(Name)
 
+#define ENSURE_TASKS_HAVE_BEEN_RUN(isolate)                                    \
+  while (v8::platform::PumpMessageLoop(v8::internal::V8::GetCurrentPlatform(), \
+                                       (isolate)))                             \
+    ;
+
 
 void RunWithProfiler(void (*test)()) {
   LocalContext env;
@@ -3275,6 +3281,7 @@ TEST(TwoPassPhantomCallbacks) {
   }
   CHECK_EQ(static_cast<int>(kLength), instance_counter);
   CcTest::heap()->CollectAllGarbage();
+  ENSURE_TASKS_HAVE_BEEN_RUN(isolate);
   CHECK_EQ(0, instance_counter);
 }
 
@@ -3293,6 +3300,7 @@ TEST(TwoPassPhantomCallbacksNestedGc) {
   array[15]->MarkTriggerGc();
   CHECK_EQ(static_cast<int>(kLength), instance_counter);
   CcTest::heap()->CollectAllGarbage();
+  ENSURE_TASKS_HAVE_BEEN_RUN(isolate);
   CHECK_EQ(0, instance_counter);
 }
 
@@ -6806,6 +6814,7 @@ THREADED_TEST(GCFromWeakCallbacks) {
                             v8::WeakCallbackType::kParameter);
       object.handle.MarkIndependent();
       invoke_gc[outer_gc]();
+      ENSURE_TASKS_HAVE_BEEN_RUN(isolate);
       CHECK(object.flag);
     }
   }