Pass NetworkMonitorFactory through PeerConnectionFactory.

Previously the instance was set through a static method, which was
really only done because it was difficult to add new
PeerConnectionFactory construction arguments at the time.

Now that we have PeerConnectionFactoryDependencies it's easy to clean
this up.

I'm doing this because I plan to add a NetworkMonitor implementation
for iOS, and don't want to inherit this ugliness.

Bug: webrtc:9883
Change-Id: Id94dc061ab1c7186b81af8547393a6e336ff04c2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/180241
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Commit-Queue: Taylor <deadbeef@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31815}

rtc_base/network_monitor.h

@@ -98,26 +98,6 @@ class NetworkMonitorBase : public NetworkMonitorInterface,
   Thread* worker_thread_;
 };
 
-/*
- * NetworkMonitorFactory creates NetworkMonitors.
- */
-class NetworkMonitorFactory {
- public:
-  // This is not thread-safe; it should be called once (or once per audio/video
-  // call) during the call initialization.
-  static void SetFactory(NetworkMonitorFactory* factory);
-
-  static void ReleaseFactory(NetworkMonitorFactory* factory);
-  static NetworkMonitorFactory* GetFactory();
-
-  virtual NetworkMonitorInterface* CreateNetworkMonitor() = 0;
-
-  virtual ~NetworkMonitorFactory();
-
- protected:
-  NetworkMonitorFactory();
-};
-
 }  // namespace rtc
 
 #endif  // RTC_BASE_NETWORK_MONITOR_H_