Rework approach to allowing extra flags for CrOS builds.

build/config/BUILDCONFIG.gn

Index: build/config/BUILDCONFIG.gn
diff --git a/build/config/BUILDCONFIG.gn b/build/config/BUILDCONFIG.gn
index 39fbf35695b4a17e2619cd1702653503daf26028..eb786f66caa3fb7550cf3d878e97088f5cd2b193 100644
--- a/build/config/BUILDCONFIG.gn
+++ b/build/config/BUILDCONFIG.gn
@@ -500,6 +500,10 @@ if (is_debug) {
   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