Correctly set -stdlib=libc++ when building with a custom libc++.

build/toolchain/gcc_toolchain.gni

 # Copyright (c) 2013 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/nacl/config.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/toolchain/cc_wrapper.gni")
 import("//build/toolchain/goma.gni")
 import("//build/toolchain/toolchain.gni")
 
@@ skipping 411 matching lines @@
       if (defined(invoker.symbol_level)) {
         symbol_level = invoker.symbol_level
       }
       if (defined(invoker.use_allocator)) {
         use_allocator = invoker.use_allocator
       }
       if (defined(invoker.use_gold)) {
         use_gold = invoker.use_gold
       }
 
-      if (defined(invoker.clear_sanitizers) && invoker.clear_sanitizers) {
-        is_asan = false
-        is_cfi = false
-        is_lsan = false
-        is_msan = false
-        is_syzyasan = false
-        is_tsan = false
-        is_ubsan = false
-        is_ubsan_vptr = false
-      }
+      # Disable sanitizers for non-default toolchains.
+      is_asan = false
+      is_cfi = false
+      is_lsan = false
+      is_msan = false
+      is_syzyasan = false
+      is_tsan = false
+      is_ubsan = false
+      is_ubsan_vptr = false
     }
 
     forward_variables_from(invoker, [ "deps" ])
   }
 }
 
 # This is a shorthand for gcc_toolchain instances based on the
 # Chromium-built version of Clang.  Only the toolchain_cpu and
 # toolchain_os variables need to be specified by the invoker, and
 # optionally toolprefix if it's a cross-compile case.  Note that for
@@ skipping 25 matching lines @@
 
     forward_variables_from(invoker,
                            [
                              "toolchain_cpu",
                              "toolchain_os",
                              "use_gold",
                              "strip",
                            ])
   }
 }