[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/rtc_base/sslidentity_unittest.cc b/rtc_base/sslidentity_unittest.cc
index ba53d17..d2ec157 100644
--- a/rtc_base/sslidentity_unittest.cc
+++ b/rtc_base/sslidentity_unittest.cc
@@ -8,17 +8,20 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
+#include <string.h>
 #include <memory>
 #include <string>
+#include <vector>
 
+#include "rtc_base/checks.h"
 #include "rtc_base/fakesslidentity.h"
-#include "rtc_base/gunit.h"
 #include "rtc_base/helpers.h"
+#include "rtc_base/logging.h"
 #include "rtc_base/messagedigest.h"
-#include "rtc_base/ssladapter.h"
 #include "rtc_base/sslfingerprint.h"
 #include "rtc_base/sslidentity.h"
 #include "rtc_base/stringutils.h"
+#include "test/gtest.h"
 
 using rtc::SSLIdentity;