Rework approach to allowing extra flags for CrOS builds.

build/config/compiler/BUILD.gn

 # 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/android/config.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/nacl/config.gni")
 import("//build/toolchain/cc_wrapper.gni")
 import("//build/toolchain/toolchain.gni")
@@ skipping 1557 matching lines @@
 
 if (is_ios || is_mac) {
   # On Mac and iOS, this enables support for ARC (automatic ref-counting).
   # See http://clang.llvm.org/docs/AutomaticReferenceCounting.html.
   config("enable_arc") {
     common_flags = [ "-fobjc-arc" ]
     cflags_objc = common_flags
     cflags_objcc = common_flags
   }
 }
+
+#
+# Extra flags
+#
+# This is a placeholder config so that users with custom toolchains
+# can more easily set extra flags to be passed to each target, while
+# still allowing for those flags to be turned off on a per-target basis.
+
+config("extra_flags") {
+  # We generally do not want people to specify extra flags, so for now
+  # this hook is only exposed to the actual CrOS builds (the simplechrome
+  # and ebuild builds, not the "desktop chromeos" build). We do that
+  # by checking for the custom_toolchain specifically, rather than
+  # just checking if `target_os == "chromeos"`.
+  if (default_toolchain == "//build/toolchain/cros:target") {
+    configs = [ "//build/config/cros:extra_flags" ]
+  }
+}