Rework approach to allowing extra flags for CrOS builds.

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 79723f6cc76c041697955f5c7de91e2935e78aa5..5f0c44215b9f0d2589e43b161e2b699a72e72fe4 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -1575,3 +1575,21 @@ if (is_ios || is_mac) {
     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" ]
+  }
+}