Run correct MicrotaskQueue on MicrotasksScope instead of the default one

MicrotasksScope has accidentally ignored the given MicrotaskQueue instance
when it's scoping out. That confused the embedder to start using the non
default MicrotaskQueue.

Change-Id: Id345605cf6520cd073429b08698de75f7681d93c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1563836Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60765}

src/api.cc

@@ -8960,7 +8960,7 @@ MicrotasksScope::~MicrotasksScope() {
   if (run_) {
     microtask_queue_->DecrementMicrotasksScopeDepth();
     if (MicrotasksPolicy::kScoped == microtask_queue_->microtasks_policy()) {
-      PerformCheckpoint(reinterpret_cast<Isolate*>(isolate_));
+      microtask_queue_->PerformCheckpoint(reinterpret_cast<Isolate*>(isolate_));
     }
   }
 #ifdef DEBUG

test/unittests/microtask-queue-unittest.cc

@@ -584,5 +584,21 @@ TEST_F(MicrotaskQueueTest, DetachGlobal_InactiveHandler) {
       Object::GetElement(isolate(), result, 1).ToHandleChecked()->IsFalse());
 }
 
+TEST_F(MicrotaskQueueTest, MicrotasksScope) {
+  ASSERT_NE(isolate()->default_microtask_queue(), microtask_queue());
+  microtask_queue()->set_microtasks_policy(MicrotasksPolicy::kScoped);
+
+  bool ran = false;
+  {
+    MicrotasksScope scope(v8_isolate(), microtask_queue(),
+                          MicrotasksScope::kRunMicrotasks);
+    microtask_queue()->EnqueueMicrotask(*NewMicrotask([&ran]() {
+      EXPECT_FALSE(ran);
+      ran = true;
+    }));
+  }
+  EXPECT_TRUE(ran);
+}
+
 }  // namespace internal
 }  // namespace v8