Migrate MessageLoop::ScopedNestableTaskAllower to...

Migrate MessageLoop::ScopedNestableTaskAllower to RunLoop::Type::kNestableTasksAllowed in /third_party/blink/renderer/platform/scheduler/child

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=alexclarke@chromium.org

Bug: 781352
Change-Id: I3ffef78836b7c4ffe22be0bc8043c39d9397729f
Reviewed-on: https://chromium-review.googlesource.com/1013216Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551251}

third_party/blink/renderer/platform/scheduler/child/idle_helper_unittest.cc

@@ -476,7 +476,6 @@ class IdleHelperWithMessageLoopTest : public BaseIdleHelperTest {
   void PostFromNestedRunloop(
       std::vector<std::pair<SingleThreadIdleTaskRunner::IdleTask, bool>>*
           tasks) {
-    base::MessageLoop::ScopedNestableTaskAllower allow(message_loop_.get());
     for (std::pair<SingleThreadIdleTaskRunner::IdleTask, bool>& pair : *tasks) {
       if (pair.second) {
         idle_task_runner_->PostIdleTask(FROM_HERE, std::move(pair.first));
@@ -488,7 +487,7 @@ class IdleHelperWithMessageLoopTest : public BaseIdleHelperTest {
     idle_helper_->StartIdlePeriod(
         IdleHelper::IdlePeriodState::kInShortIdlePeriod, clock_.NowTicks(),
         clock_.NowTicks() + base::TimeDelta::FromMilliseconds(10));
-    base::RunLoop().RunUntilIdle();
+    base::RunLoop(base::RunLoop::Type::kNestableTasksAllowed).RunUntilIdle();
   }
 
   void SetUp() override {