Move libc++ configs and flags out of //build/config/sanitizers

build/config/c++/BUILD.gn

Index: build/config/c++/BUILD.gn
diff --git a/build/config/c++/BUILD.gn b/build/config/c++/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..21316ba1c8bd3160c4b08a0a65bea5b72871e6c9
--- /dev/null
+++ b/build/config/c++/BUILD.gn
@@ -0,0 +1,17 @@
+# 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/c++/c++.gni")
+
+config("c++flags") {
+  if (use_custom_libcxx) {
+    prefix = "//buildtools/third_party"
+    include = "trunk/include"
+    cflags_cc = [
+      "-nostdinc++",
+      "-isystem" + rebase_path("$prefix/libc++/$include", root_build_dir),
+      "-isystem" + rebase_path("$prefix/libc++abi/$include", root_build_dir),
+    ]
+  }
+}