Directly enable -Wglobal-constructors

Relying on the config inside //build breaks in projects that use their
own version of //build instead of Chromium's.

Bug: dawn:1405
Change-Id: I18bbf5c6ddce18e6900f5f4b937d91a152bb2b32
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/90524
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>

src/dawn/native/BUILD.gn

@@ -157,12 +157,14 @@ source_set("sources") {
 
   configs += [
     ":internal",
-
-    # Enable -Wglobal-constructors here only, instead of in internal_config,
-    # because gtest and some other targets don't build with it.
-    "//build/config/compiler:wglobal_constructors",
   ]
 
+  # Enable -Wglobal-constructors here only, instead of in internal_config,
+  # because gtest and some other targets don't build with it.
+  if (is_clang) {
+    cflags = [ "-Wglobal-constructors" ]
+  }
+
   # Dependencies that are needed to compile dawn native entry points in
   # FooBackend.cpp need to be public deps so they are propagated to the
   # dawn native target