Use fake clock in some more networks tests.

webrtc/base/signalthread.h

Index: webrtc/base/signalthread.h
diff --git a/webrtc/base/signalthread.h b/webrtc/base/signalthread.h
index a91e4ad34ae8162175edaba062b7e1b3d3db798e..d42d6efd59341e594f3f895afc437aaee94755d4 100644
--- a/webrtc/base/signalthread.h
+++ b/webrtc/base/signalthread.h
@@ -111,6 +111,7 @@ class SignalThread
           parent_(parent) {}
     ~Worker() override;
     void Run() override;
+    bool IsProcessingMessages() override;
 
    private:
     SignalThread* parent_;