Change ObjCNetworkMonitor::OnPathUpdate to use PostTask

Removes use of AsyncInvoker, replaced with PendingTaskSafetyFlag. The
latter is extended to support creation on a different thread than
where it will be used, and to support stop and restart.

Bug: webrtc:12339
Change-Id: I28b6e09b1542f50037e842ef5fe3a47d15704b46
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/211002
Commit-Queue: Niels Moller <nisse@webrtc.org>
Reviewed-by: Tommi <tommi@webrtc.org>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33432}

rtc_base/task_utils/pending_task_safety_flag.cc

@@ -24,6 +24,11 @@ void PendingTaskSafetyFlag::SetNotAlive() {
   alive_ = false;
 }
 
+void PendingTaskSafetyFlag::SetAlive() {
+  RTC_DCHECK_RUN_ON(&main_sequence_);
+  alive_ = true;
+}
+
 bool PendingTaskSafetyFlag::alive() const {
   RTC_DCHECK_RUN_ON(&main_sequence_);
   return alive_;

rtc_base/task_utils/pending_task_safety_flag.h

@@ -62,6 +62,7 @@ class PendingTaskSafetyFlag : public rtc::RefCountInterface {
   ~PendingTaskSafetyFlag() = default;
 
   void SetNotAlive();
+  void SetAlive();
   bool alive() const;
 
  protected: