Reland "Change SimpleStringBuilder::Append to not use strcpyn and SIZE_UNKNOWN"

This is a reland of e779847fb6499ac2dc4757de8c625ac377e9d0d4

Original change's description:
> Change SimpleStringBuilder::Append to not use strcpyn and SIZE_UNKNOWN
>
> Also add explicit includes of rtc_base/string_utils.h in files depending on it.
>
> Bug: webrtc:6424
> Change-Id: Id6b53937ab2d185d092a5d8863018fd5f1a88e27
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/135744
> Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
> Commit-Queue: Niels Moller <nisse@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#27903}

Tbr: kwiberg@webrtc.org
Bug: webrtc:6424
Change-Id: Ic08d5d7fbc25ff89e4182d7c9cb3b0e8e356339a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/135946
Reviewed-by: Niels Moller <nisse@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27957}
diff --git a/sdk/android/native_unittests/stacktrace/stacktrace_unittest.cc b/sdk/android/native_unittests/stacktrace/stacktrace_unittest.cc
index 4ea3c9f..917dbc9 100644
--- a/sdk/android/native_unittests/stacktrace/stacktrace_unittest.cc
+++ b/sdk/android/native_unittests/stacktrace/stacktrace_unittest.cc
@@ -19,6 +19,7 @@
 #include "rtc_base/event.h"
 #include "rtc_base/logging.h"
 #include "rtc_base/platform_thread.h"
+#include "rtc_base/string_utils.h"
 #include "rtc_base/strings/string_builder.h"
 #include "rtc_base/system/inline.h"
 #include "system_wrappers/include/sleep.h"