[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}

modules/video_coding/generic_encoder_unittest.cc

@@ -8,12 +8,14 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
+#include <cstddef>
 #include <vector>
 
-#include "modules/video_coding/encoded_frame.h"
+#include "api/video/video_timing.h"
 #include "modules/video_coding/generic_encoder.h"
 #include "modules/video_coding/include/video_coding_defines.h"
 #include "rtc_base/fakeclock.h"
+#include "rtc_base/timeutils.h"
 #include "test/gtest.h"
 
 namespace webrtc {