[cleanup] Use the new taskrunner API for idle tasks in the GC

Like https://crrev.com/c/1261145, but I forgot one use in the scavenger.

R=ulan@chromium.org

Bug: v8:8238
Change-Id: I75a1e1ef8d2bb7dddfc648c0a5b827ad466f0ad3
Reviewed-on: https://chromium-review.googlesource.com/c/1261935Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56398}

src/heap/scavenge-job.cc

@@ -107,8 +107,9 @@ void ScavengeJob::ScheduleIdleTask(Heap* heap) {
     v8::Isolate* isolate = reinterpret_cast<v8::Isolate*>(heap->isolate());
     if (V8::GetCurrentPlatform()->IdleTasksEnabled(isolate)) {
       idle_task_pending_ = true;
-      auto task = new IdleTask(heap->isolate(), this);
-      V8::GetCurrentPlatform()->CallIdleOnForegroundThread(isolate, task);
+      auto task = base::make_unique<IdleTask>(heap->isolate(), this);
+      V8::GetCurrentPlatform()->GetForegroundTaskRunner(isolate)->PostIdleTask(
+          std::move(task));
     }
   }
 }