Rework approach to allowing extra flags for CrOS builds.

build/config/cros/BUILD.gn

+# Copyright 2016 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/toolchain/cros_toolchain.gni")
+
+config("extra_flags") {
+  if (current_toolchain == "//build/toolchain/cros:target") {
+    cflags_c = [ cros_target_extra_cflags ]
+    cflags = [ cros_target_extra_cppflags ]
+    cflags_cc = [ cros_target_extra_cxxflags ]
+    ldflags = [ cros_target_extra_ldflags ]
+  } else if (current_toolchain == "//build/toolchain/cros:host") {
+    cflags_c = [ cros_host_extra_cflags ]
+    cflags = [ cros_host_extra_cppflags ]
+    cflags_cc = [ cros_host_extra_cxxflags ]
+    ldflags = [ cros_host_extra_ldflags ]
+  } else if (current_toolchain == "//build/toolchain/cros:v8_snapshot") {
+    cflags_c = [ cros_v8_snapshot_extra_cflags ]
+    cflags = [ cros_v8_snapshot_extra_cppflags ]
+    cflags_cc = [ cros_v8_snapshot_extra_cxxflags ]
+    ldflags = [ cros_v8_snapshot_extra_ldflags ]
+  } else if (current_toolchain == "//build/toolchain/cros:nacl_bootstrap") {
+    cflags_c = [ cros_nacl_bootstrap_extra_cflags ]
+    cflags = [ cros_nacl_bootstrap_extra_cppflags ]
+    cflags_cc = [ cros_nacl_bootstrap_extra_cxxflags ]
+    ldflags = [ cros_nacl_bootstrap_extra_ldflags ]
+  }
+}