Add simulated time support for PC level test.

Bug: webrtc:11743
Change-Id: If69ab07618a30ec1a66dd5f36b3198486bee55fa
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/178608
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Andrey Logvin <landrey@webrtc.org>
Reviewed-by: Tommi <tommi@webrtc.org>
Commit-Queue: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31639}
diff --git a/api/test/time_controller.h b/api/test/time_controller.h
index 4d7f9e6..419bd20 100644
--- a/api/test/time_controller.h
+++ b/api/test/time_controller.h
@@ -46,6 +46,7 @@
       const char* thread_name) = 0;
   // Creates an rtc::Thread instance. If |socket_server| is nullptr, a default
   // noop socket server is created.
+  // Returned thread is not null and started.
   virtual std::unique_ptr<rtc::Thread> CreateThread(
       const std::string& name,
       std::unique_ptr<rtc::SocketServer> socket_server = nullptr) = 0;