Migrate MessageLoop::ScopedNestableTaskAllower to RunLoop::Type::kNestableTasksAllowed in /base

This pass focuses on ML::SNTA in same scope as an anonymous RunLoop().RunUntilIdle()
Scripted via a small tweak of https://bugs.chromium.org/p/chromium/issues/detail?id=781352#c1
which focused on RunLoops with a variable name.

The script should have replaced a MessageLoop::ScopedNestableTaskAllower
in the same scope as a RunLoop with a RunLoop of
Type::kNestableTasksAllowed. If there was a comment above the
MessageLoop::ScopedNestableTaskAllower, it should have been moved
alongside the augmented RunLoop.

Includes should have been stripped if the was the last usage of message_loop.h
in that file.

Please CQ if LGTY

This CL was uploaded by git cl split.

R=thakis@chromium.org

Bug: 781352
Change-Id: I0cb8cd0bbe6161bc706a5fc81d73c1d41ddcc775
Reviewed-on: https://chromium-review.googlesource.com/995974
Commit-Queue: Gabriel Charette <gab@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548141}

base/message_loop/message_loop_unittest.cc

@@ -1257,10 +1257,7 @@ namespace {
 
 void FuncThatPumps(TaskList* order, int cookie) {
   order->RecordStart(PUMPS, cookie);
-  {
-    MessageLoop::ScopedNestableTaskAllower allow(MessageLoop::current());
-    RunLoop().RunUntilIdle();
-  }
+  RunLoop(RunLoop::Type::kNestableTasksAllowed).RunUntilIdle();
   order->RecordEnd(PUMPS, cookie);
 }