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/modules/audio_mixer/audio_mixer_impl.h b/modules/audio_mixer/audio_mixer_impl.h
index e8994a6..8edd3b8 100644
--- a/modules/audio_mixer/audio_mixer_impl.h
+++ b/modules/audio_mixer/audio_mixer_impl.h
@@ -17,6 +17,7 @@
 #include "api/audio/audio_mixer.h"
 #include "modules/audio_mixer/frame_combiner.h"
 #include "modules/audio_mixer/output_rate_calculator.h"
+#include "rtc_base/criticalsection.h"
 #include "rtc_base/race_checker.h"
 #include "rtc_base/scoped_ref_ptr.h"
 #include "rtc_base/thread_annotations.h"
diff --git a/modules/congestion_controller/rtp/send_side_congestion_controller.cc b/modules/congestion_controller/rtp/send_side_congestion_controller.cc
index 266a5dc..3970f13 100644
--- a/modules/congestion_controller/rtp/send_side_congestion_controller.cc
+++ b/modules/congestion_controller/rtp/send_side_congestion_controller.cc
@@ -21,6 +21,7 @@
 #include "modules/remote_bitrate_estimator/include/bwe_defines.h"
 #include "rtc_base/bind.h"
 #include "rtc_base/checks.h"
+#include "rtc_base/event.h"
 #include "rtc_base/format_macros.h"
 #include "rtc_base/logging.h"
 #include "rtc_base/numerics/safe_conversions.h"
diff --git a/modules/rtp_rtcp/source/contributing_sources.h b/modules/rtp_rtcp/source/contributing_sources.h
index 1a4a572..b6201ce 100644
--- a/modules/rtp_rtcp/source/contributing_sources.h
+++ b/modules/rtp_rtcp/source/contributing_sources.h
@@ -18,7 +18,8 @@
 
 #include "absl/types/optional.h"
 #include "api/array_view.h"
-#include "api/rtpreceiverinterface.h"
+#include "api/rtpreceiverinterface.h"  // For RtpSource
+#include "rtc_base/timeutils.h"        // For kNumMillisecsPerSec
 
 namespace webrtc {