[MessageLoop]: Replace RunLoop-related ScopedNestableTaskAllower in /base.

This CL replaces ScopedNestableTaskAllower with kNestableTasksAllowed passed
as argument to the RunLoop involved if any.
Only native nested loops should use ScopedNestableTaskAllower (soon to be deprecated).
For more details, see:
https://cs.chromium.org/chromium/src/base/message_loop/message_loop_current.h?sq=package:chromium&dr=CSs&g=0&l=159

A small step towards deleting MessageLoop.

This CL was generated with git cl split.
Refer to the top-level CL if necessary :
https://chromium-review.googlesource.com/c/chromium/src/+/1635819

Please CQ if LGTY!

This CL was uploaded by git cl split.

R=thestig@chromium.org

Bug: 766678
Change-Id: I6624b7e20ad6d7852fa91151ddd149932cebe516
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1636056
Auto-Submit: Etienne Pierre-Doray <etiennep@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664806}

base/test/scoped_task_environment.cc

@@ -594,12 +594,11 @@ void ScopedTaskEnvironment::RunUntilIdle() {
 }
 
 void ScopedTaskEnvironment::FastForwardBy(TimeDelta delta) {
-  MessageLoopCurrent::ScopedNestableTaskAllower allow;
   DCHECK(mock_time_domain_);
   mock_time_domain_->SetStopWhenMessagePumpIsIdle(false);
   mock_time_domain_->SetAllowTimeToAutoAdvanceUntil(mock_time_domain_->Now() +
                                                     delta);
-  RunLoop().RunUntilIdle();
+  RunLoop{RunLoop::Type::kNestableTasksAllowed}.RunUntilIdle();
   mock_time_domain_->SetStopWhenMessagePumpIsIdle(true);
   mock_time_domain_->SetAllowTimeToAutoAdvanceUntil(TimeTicks::Max());
 }