Make rtc_base/fake_mdns_responder.h self contained.

The header "rtc_base/fake_mdns_responder.h" was not self contained,
and it was relying on order includes to get all the types it needs.

This CL makes it self-contained and removes an unneeded #include.

Bug: None
Change-Id: Ie6c584a169ef884d79e436e51c2e72236b0d4c7a
Reviewed-on: https://webrtc-review.googlesource.com/c/125184
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26938}
diff --git a/rtc_base/fake_mdns_responder.h b/rtc_base/fake_mdns_responder.h
index 17a7678..4290876 100644
--- a/rtc_base/fake_mdns_responder.h
+++ b/rtc_base/fake_mdns_responder.h
@@ -15,9 +15,11 @@
 #include <memory>
 #include <string>
 
+#include "rtc_base/async_invoker.h"
+#include "rtc_base/ip_address.h"
+#include "rtc_base/location.h"
 #include "rtc_base/mdns_responder_interface.h"
-
-#include "rtc_base/helpers.h"
+#include "rtc_base/thread.h"
 
 namespace webrtc {