Define enable_safe_libcxx in build_overrides/build.gni for standalone ipcz.

enable_safe_libcxx will be overridable by projects that embed Chrome's
//build using the build_overrides mechanism. All downstream projects
will need to define this new variable so Chrome can stop conditionally
defining enable_safe_libcxx upstream.

Bug: chromium:1385662
Change-Id: I7a7d2a9b3099a87c36dc7f0ebf85962d4ec43bbe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4265594
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1107125}
NOKEYCHECK=True
GitOrigin-RevId: 810eb4c62851356704f4cfd731975f3728b5fb06
diff --git a/build_overrides/build.gni b/build_overrides/build.gni
index be40719..de7c2db 100644
--- a/build_overrides/build.gni
+++ b/build_overrides/build.gni
@@ -4,6 +4,9 @@
 
 import("//build/config/gclient_args.gni")
 
+# Enables assertions on safety checks in libc++.
+enable_safe_libcxx = true
+
 declare_args() {
   gtest_enable_absl_printers = true
   ignore_elf32_limitations = false