Introduce RunLoop::Type::NESTABLE_TASKS_ALLOWED to replace MessageLoop::ScopedNestableTaskAllower.

(as well as MessageLoop::SetNestableTasksAllowed())

Surveying usage: the scoped object is always instantiated right before
RunLoop().Run(). The intent is really to allow nestable tasks in that
RunLoop so it's better to explicitly label that RunLoop as such and it
allows us to break the last dependency that forced some RunLoop users
to use MessageLoop APIs.

There's also the odd case of allowing nestable tasks for loops that are
reentrant from a native task (without going through RunLoop), these
are the minority but will have to be handled (after cleaning up the
majority of cases that are RunLoop induced).
As highlighted by robliao@ in https://chromium-review.googlesource.com/c/600517
(which was merged in this CL).

R=danakj@chromium.org

Bug: 750779
Change-Id: I43d122c93ec903cff3a6fe7b77ec461ea0656448
Reviewed-on: https://chromium-review.googlesource.com/594713
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#492263}
void MessageLoop::ReloadWorkQueue() {
  if (work_queue_.empty()) {
#if defined(OS_WIN)
    pending_high_res_tasks_ +=
        incoming_task_queue_->ReloadWorkQueue(&work_queue_);
#else
    incoming_task_queue_->ReloadWorkQueue(&work_queue_);
#endif
  }
}

void MessageLoop::ReloadWorkQueue() {
  if (work_queue_.empty()) {
#if defined(OS_WIN)
    pending_high_res_tasks_ +=
        incoming_task_queue_->ReloadWorkQueue(&work_queue_);
#else
    incoming_task_queue_->ReloadWorkQueue(&work_queue_);
#endif
  }
}
