[scheduler] Run tasks after frame detach

Stop deleting tasks in task queues upon frame detach.

R=dcheng@chromium.org,hajimehoshi@chromium.org,alexclarke@chromium.org
BUG=770736

Change-Id: I8c0a7f6da2bd383bddf32789a1c14d1a64d2b959
Reviewed-on: https://chromium-review.googlesource.com/800943Reviewed-by: Hajime Hoshi <hajimehoshi@chromium.org>
Commit-Queue: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520908}

third_party/WebKit/Source/platform/scheduler/renderer/web_frame_scheduler_impl.cc

@@ -114,9 +114,10 @@ namespace {
 void CleanUpQueue(MainThreadTaskQueue* queue) {
   if (!queue)
     return;
-  queue->ShutdownTaskQueue();
+  queue->DetachFromRendererScheduler();
   queue->SetFrameScheduler(nullptr);
   queue->SetBlameContext(nullptr);
+  queue->SetQueuePriority(TaskQueue::QueuePriority::kLowPriority);
 }
 
 }  // namespace

third_party/WebKit/Source/platform/scheduler/renderer/web_view_scheduler_impl_unittest.cc

@@ -483,15 +483,14 @@ TEST_F(WebViewSchedulerImplTest, DeleteThrottledQueue_InTask) {
       BLINK_FROM_HERE, MakeRepeatingTask(timer_task_queue, &run_count),
       TimeDelta::FromMilliseconds(1));
 
-  // Note this will run at time t = 10s since we start at time t = 5000us, and
-  // it will prevent further tasks from running (i.e. the RepeatingTask) by
-  // deleting the WebFrameScheduler.
+  // Note this will run at time t = 10s since we start at time t = 5000us.
+  // However, we still should run all tasks after frame scheduler deletion.
   timer_task_queue->PostDelayedTask(BLINK_FROM_HERE,
                                     MakeDeletionTask(web_frame_scheduler),
                                     TimeDelta::FromMilliseconds(9990));
 
   mock_task_runner_->RunForPeriod(base::TimeDelta::FromSeconds(100));
-  EXPECT_EQ(10, run_count);
+  EXPECT_EQ(90015, run_count);
 }
 
 TEST_F(WebViewSchedulerImplTest, VirtualTimePauseCount_DETERMINISTIC_LOADING) {