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/rtc_base/strings/string_builder.cc b/rtc_base/strings/string_builder.cc
index 73d763a..a204e52 100644
--- a/rtc_base/strings/string_builder.cc
+++ b/rtc_base/strings/string_builder.cc
@@ -26,7 +26,7 @@
 }
 
 SimpleStringBuilder& SimpleStringBuilder::operator<<(const char* str) {
-  return Append(str);
+  return Append(str, strlen(str));
 }
 
 SimpleStringBuilder& SimpleStringBuilder::operator<<(char ch) {
@@ -107,11 +107,12 @@
 
 SimpleStringBuilder& SimpleStringBuilder::Append(const char* str,
                                                  size_t length) {
-  const size_t chars_added =
-      rtc::strcpyn(&buffer_[size_], buffer_.size() - size_, str, length);
-  size_ += chars_added;
-  RTC_DCHECK_EQ(chars_added, length == SIZE_UNKNOWN ? std::strlen(str) : length)
+  RTC_DCHECK_LT(size_ + length, buffer_.size())
       << "Buffer size was insufficient";
+  const size_t chars_added = rtc::SafeMin(length, buffer_.size() - size_ - 1);
+  memcpy(&buffer_[size_], str, chars_added);
+  size_ += chars_added;
+  buffer_[size_] = '\0';
   RTC_DCHECK(IsConsistent());
   return *this;
 }