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 79c79a42d21ad8fdb6f807ab5ae6990280b20693..d3b63e5c4505fceca9b645a58b5049b06aa9d3a7 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -1577,3 +1577,18 @@ 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.
+# We generally do not want people to specify extra flags, so for now
+# this hook is only exposed to the CrOS build.
+
+config("extra_flags") {
+  if (default_toolchain == "//build/config/cros:target") {
+    configs = [ "//build/toolchain/cros:extra_flags" ]
+  }

[Dirk Pranke, 2016/08/02 00:28:10]

Perhaps instead of hardcoding the toolchain and config names, this should be a
build arg instead?

[brettw, 2016/08/03 23:02:14]

Can this just be:
  if (target_os == "chromeos") ?

[Dirk Pranke, 2016/08/03 23:09:07]

No, because that would break the "desktop chromeos" config,
where we build using the chromium checked-in clang toolchain.

Ideally some day we'd separate out "want chromeos ui and functionality" from
"want to run on the chromeos operating system", in which case this distinction
would be a lot clearer.

[brettw, 2016/08/08 22:10:56]

Ah, then can you add an explicit comment explaining this? It's very subtle.

+}