Rework approach to allowing extra flags for CrOS builds.

build/config/BUILDCONFIG.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.
 
 # =============================================================================
 # PLATFORM SELECTION
 # =============================================================================
 #
 # There are two main things to set: "os" and "cpu". The "toolchain" is the name
 # of the GN thing that encodes combinations of these things.
@@ skipping 482 matching lines @@
   ]
 }
 
 # Debug/release-related defines.
 if (is_debug) {
   default_compiler_configs += [ "//build/config:debug" ]
 } else {
   default_compiler_configs += [ "//build/config:release" ]
 }
 
+# Lastly, add a setting to give toolchains an extra chance to override
+# flags as needed.
+default_compiler_configs += [ "//build/config/compiler:extra_flags" ]
+
 # Static libraries and source sets use only the compiler ones.
 set_defaults("static_library") {
   configs = default_compiler_configs
 }
 set_defaults("source_set") {
   configs = default_compiler_configs
 }
 
 # Compute the set of configs common to all linked targets (shared libraries,
 # loadable modules, executables) to avoid duplication below.
@@ skipping 109 matching lines @@
 set_defaults("component") {
   if (is_component_build) {
     configs = default_shared_library_configs
     if (is_android) {
       configs -= [ "//build/config/android:hide_native_jni_exports" ]
     }
   } else {
     configs = default_compiler_configs
   }
 }