Reland "[cleanup] Remove useless includes."

Reason for reland: Downstream project fixed.

Original change's description:

> [cleanup] Remove useless includes.
>
> Manual cleanup guided by include-what-you-use diagnostic.
>
> Bug: webrtc:8311
> Change-Id: I00be03392cc7ee005101427ea7dc701621ccea68
> Reviewed-on: https://webrtc-review.googlesource.com/c/103320
> Commit-Queue: Yves Gerey <yvesg@webrtc.org>
> Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#25013}

Bug: webrtc:8311
Change-Id: Id6ec4aeb798886a90ace640a190eaf16497ba31b
Reviewed-on: https://webrtc-review.googlesource.com/c/104120
Commit-Queue: Yves Gerey <yvesg@webrtc.org>
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25034}
diff --git a/rtc_base/sequenced_task_checker_unittest.cc b/rtc_base/sequenced_task_checker_unittest.cc
index 96e655b..83fb14f 100644
--- a/rtc_base/sequenced_task_checker_unittest.cc
+++ b/rtc_base/sequenced_task_checker_unittest.cc
@@ -12,6 +12,7 @@
 
 #include "rtc_base/checks.h"
 #include "rtc_base/constructormagic.h"
+#include "rtc_base/event.h"
 #include "rtc_base/platform_thread.h"
 #include "rtc_base/task_queue.h"
 #include "rtc_base/thread_checker.h"