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}

sdk/BUILD.gn

@@ -1560,6 +1560,8 @@ if (is_ios || is_mac) {
           "../api:sequence_checker",
           "../rtc_base",
           "../rtc_base:threading",
+          "../rtc_base/task_utils:pending_task_safety_flag",
+          "../rtc_base/task_utils:to_queued_task",
         ]
       }
     }