Simplify condition to use libc++

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

Index: build/config/c++/c++.gni
diff --git a/build/config/c++/c++.gni b/build/config/c++/c++.gni
index 5a62670afafef626d726682de03ba524f2a956c3..eca1b2cdd02fc1299744b0136ca43a003c9fe56a 100644
--- a/build/config/c++/c++.gni
+++ b/build/config/c++/c++.gni
@@ -4,20 +4,10 @@
 
 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_tsan || is_msan || is_ubsan || is_ubsan_security ||
-      (use_libfuzzer && !is_mac) || use_afl
+      is_linux && !is_chromeos && (!is_chromecast || is_cast_desktop_build)
 }