[scheduler] Do not lower priority for tasks on detached frames

Frames that are detached may still have tasks scheduled to run. These
tasks may hold unnecessary state and memory until they are run.

While this may generally be not ideal, it also breaks testing
infrastructure such as leak detection that assumes all cleanup tasks are
run at some point. In order to guaranteee that tasks have a chance to
run the leak detector schedules itself repeatedly. Since the tasks on
the detached frames are low priority, there's no guarantee that they are
run in time.

Bug: 936360, 843903
Change-Id: Ifd4ec86beb9b27b4555d792c2b4df1336474ead1
Reviewed-on: https://chromium-review.googlesource.com/c/1491412
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635969}

third_party/blink/renderer/platform/scheduler/main_thread/frame_scheduler_impl.cc

@@ -245,7 +245,6 @@ void CleanUpQueue(MainThreadTaskQueue* queue) {
   queue->DetachFromMainThreadScheduler();
   queue->DetachFromFrameScheduler();
   queue->SetBlameContext(nullptr);
-  queue->SetQueuePriority(TaskQueue::QueuePriority::kLowPriority);
 }
 
 }  // namespace