Allow injection of time controller to NetworkEmulationManagerImpl.

Bug: webrtc:10365
Change-Id: I6a0e04459f75e8134787e605057dcb03cae55cd8
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/132780
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27631}

test/time_controller/real_time_controller.h

@@ -25,6 +25,8 @@ class RealTimeController : public TimeController {
   void InvokeWithControlledYield(std::function<void()> closure) override;
 };
 
+RealTimeController* GlobalRealTimeController();
+
 }  // namespace webrtc
 
 #endif  // TEST_TIME_CONTROLLER_REAL_TIME_CONTROLLER_H_