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
index 96e23facf844d224e039816e3e524d8cb27c6fc3..b84d18d726866883aaeb67933054b467d99cc4f9 100644
--- a/build/config/sanitizers/sanitizers.gni
+++ b/build/config/sanitizers/sanitizers.gni
@@ -12,4 +12,9 @@ declare_args() {
   # standard system libraries. Set this flag to download prebuilt binaries from
   # GCS.
   use_prebuilt_instrumented_libraries = false
+
+  # 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
 }