[Cleanup] Add missing #include. Remove useless ones. IWYU part 2.

This is a follow-up to
https://webrtc-review.googlesource.com/c/src/+/106280.
This time the whole code base is covered.
Some files may have not been fixed though, whenever the IWYU tool
was breaking the build.

Bug: webrtc:8311
Change-Id: I2c31f552a87e887d33931d46e87b6208b1e483ef
Reviewed-on: https://webrtc-review.googlesource.com/c/111965
Commit-Queue: Yves Gerey <yvesg@google.com>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25830}
diff --git a/video/video_loopback.cc b/video/video_loopback.cc
index 4cdddb9..5080c63 100644
--- a/video/video_loopback.cc
+++ b/video/video_loopback.cc
@@ -9,7 +9,17 @@
  */
 
 #include <stdio.h>
+#include <memory>
+#include <string>
+#include <vector>
 
+#include "absl/memory/memory.h"
+#include "absl/types/optional.h"
+#include "api/bitrate_constraints.h"
+#include "api/test/simulated_network.h"
+#include "api/test/video_quality_test_fixture.h"
+#include "api/video_codecs/video_codec.h"
+#include "rtc_base/checks.h"
 #include "rtc_base/flags.h"
 #include "rtc_base/logging.h"
 #include "system_wrappers/include/field_trial.h"