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

This is another scripted pass done by a slightly modified version of
https://bugs.chromium.org/p/chromium/issues/detail?id=781352#c1 which
has a better notion of "scope".

The script should have replaced a MessageLoop::ScopedNestableTaskAllower
in the same scope as a RunLoop with a RunLoop of
Type::kNestableTasksAllowed.

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

Please CQ if LGTY

This CL was uploaded by git cl split.

R=rvera@chromium.org

Bug: 781352
Change-Id: I5429418614b13b654af86c4764ac1c9066a21f95
Reviewed-on: https://chromium-review.googlesource.com/1014780Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551485}

headless/test/headless_render_test.cc

@@ -226,7 +226,7 @@ void HeadlessRenderTest::RunDevTooledTest() {
 
   // Pause virtual time until we actually start loading content.
   {
-    base::RunLoop run_loop;
+    base::RunLoop run_loop(base::RunLoop::Type::kNestableTasksAllowed);
     devtools_client_->GetEmulation()->GetExperimental()->SetVirtualTimePolicy(
         emulation::SetVirtualTimePolicyParams::Builder()
             .SetPolicy(emulation::VirtualTimePolicy::PAUSE)
@@ -240,18 +240,14 @@ void HeadlessRenderTest::RunDevTooledTest() {
             .Build(),
         base::BindOnce(&SetVirtualTimePolicyDoneCallback, &run_loop));
 
-    base::MessageLoop::ScopedNestableTaskAllower nest_loop(
-        base::MessageLoop::current());
     run_loop.Run();
   }
 
   {
-    base::RunLoop run_loop;
+    base::RunLoop run_loop(base::RunLoop::Type::kNestableTasksAllowed);
     // Note AdditionalVirtualTimeBudget will self delete.
     new AdditionalVirtualTimeBudget(virtual_time_controller_.get(), this,
                                     &run_loop, 5000);
-    base::MessageLoop::ScopedNestableTaskAllower nest_loop(
-        base::MessageLoop::current());
     run_loop.Run();
   }