Revert of Simplify condition to use libc++

build/config/c++/c++.gni

 # Copyright 2017 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/sanitizers/sanitizers.gni")
 
+if (current_toolchain != default_toolchain) {
+  use_custom_libcxx = false
+}
+
 declare_args() {
   # Use libc++ (buildtools/third_party/libc++ and
   # buildtools/third_party/libc++abi) instead of stdlibc++ as standard
   # library.
+  # TODO(thomasanderson): Clean up this complex condition.  The
+  # is_tsan, is_msan, and (use_libfuzzer && !is_mac), clauses can be
+  # removed since they are only used on non-CrOs Linux.  is_ubsan can
+  # probably also be removed for the same reason.
   use_custom_libcxx =
-      is_linux && !is_chromeos && (!is_chromecast || is_cast_desktop_build)
+      (is_linux && !is_chromeos && (!is_chromecast || is_cast_desktop_build)) ||
+      is_tsan || is_msan || is_ubsan || is_ubsan_security ||
+      (use_libfuzzer && !is_mac) || use_afl
 }