[base] Add a trace event for ScopedNestableTaskAllower

This will help identify intentional nested loops in traces.

R=fdoray@chromium.org

Bug: 899897, 1074019
Change-Id: Ie8d7522c66b00a7b7f2e94e8143a980afd48f47a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2220631Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#773311}

base/message_loop/message_loop_current.cc

@@ -93,11 +93,13 @@ bool MessageLoopCurrent::NestableTasksAllowed() const {
 MessageLoopCurrent::ScopedNestableTaskAllower::ScopedNestableTaskAllower()
     : sequence_manager_(GetCurrentSequenceManagerImpl()),
       old_state_(sequence_manager_->IsTaskExecutionAllowed()) {
+  TRACE_EVENT_BEGIN0("base", "ScopedNestableTaskAllower");
   sequence_manager_->SetTaskExecutionAllowed(true);
 }
 
 MessageLoopCurrent::ScopedNestableTaskAllower::~ScopedNestableTaskAllower() {
   sequence_manager_->SetTaskExecutionAllowed(old_state_);
+  TRACE_EVENT_END0("base", "ScopedNestableTaskAllower");
 }
 
 bool MessageLoopCurrent::operator==(const MessageLoopCurrent& other) const {