scheduler: Don't run multiple tasks per batch in a nested run loop

components/scheduler/child/task_queue_manager.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/scheduler/child/task_queue_manager.h"
 
 #include <queue>
 #include <set>
 
 #include "base/bind.h"
@@ skipping 137 matching lines @@
     // already pending, so it is safe to call it in a loop.
     MaybePostDoWorkOnMainRunner();
 
     if (ProcessTaskFromWorkQueue(queue, &previous_task))
       return;  // The TaskQueueManager got deleted, we must bail out.
 
     bool should_trigger_wakeup = queue->wakeup_policy() ==
                                  TaskQueue::WakeupPolicy::CAN_WAKE_OTHER_QUEUES;
     if (!UpdateWorkQueues(should_trigger_wakeup, &previous_task))
       return;
+
+    // Only run a single task per batch in nested run loops so that we can
+    // properly exit the nested loop when someone calls RunLoop::Quit().
+    if (main_task_runner_->IsNested())
+      break;
   }
 }
 
 bool TaskQueueManager::SelectQueueToService(
     internal::TaskQueueImpl** out_queue) {
   bool should_run = selector_.SelectQueueToService(out_queue);
   TRACE_EVENT_OBJECT_SNAPSHOT_WITH_ID(
       disabled_by_default_tracing_category_, "TaskQueueManager", this,
       AsValueWithSelectorResult(should_run, *out_queue));
   return should_run;
@@ skipping 107 matching lines @@
     state->SetString("selected_queue", selected_queue->GetName());
   return state;
 }
 
 void TaskQueueManager::OnTaskQueueEnabled() {
   DCHECK(main_thread_checker_.CalledOnValidThread());
   MaybePostDoWorkOnMainRunner();
 }
 
 }  // namespace scheduler