GN: Build with a custom libc++ when using ASAN, TSAN or MSAN.

build/config/sanitizers/sanitizers.gni

Index: build/config/sanitizers/sanitizers.gni
diff --git a/build/config/sanitizers/sanitizers.gni b/build/config/sanitizers/sanitizers.gni
new file mode 100644
index 0000000000000000000000000000000000000000..e9aeeae0492763879637d8b93e6558282e5fbab8
--- /dev/null
+++ b/build/config/sanitizers/sanitizers.gni
@@ -0,0 +1,10 @@
+# Copyright 2015 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.
+
+declare_args() {
+  # Use libc++ (buildtools/third_party/libc++ and
+  # buildtools/third_party/libc++abi) instead of stdlibc++ as standard library.
+  # This is intended to be used for instrumented builds.
+  use_custom_libcxx = (is_asan && is_linux) || is_tsan || is_msan
+}