[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/system_wrappers/source/metrics_default_unittest.cc b/system_wrappers/source/metrics_default_unittest.cc
index b2d2023..cb427c2 100644
--- a/system_wrappers/source/metrics_default_unittest.cc
+++ b/system_wrappers/source/metrics_default_unittest.cc
@@ -8,6 +8,12 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
+#include <map>
+#include <memory>
+#include <string>
+#include <utility>
+
+#include "rtc_base/checks.h"
 #include "system_wrappers/include/metrics.h"
 #include "test/gtest.h"