Update existing uses of /wd4267 to use the GN config

extensions/BUILD.gn

Index: extensions/BUILD.gn
diff --git a/extensions/BUILD.gn b/extensions/BUILD.gn
index b1e69d27a601735725701718146a837e2170080a..202469a8fce45fde6836de1f5e623b899e017e1e 100644
--- a/extensions/BUILD.gn
+++ b/extensions/BUILD.gn
@@ -70,6 +70,8 @@ source_set("test_support") {
   sources = rebase_path(extensions_gypi_values.extensions_test_support_sources,
                         ".",
                         "//extensions")
+  # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
+  configs += [ "//build/config/compiler:no_size_t_to_int_warning" ]
 
   deps = [
     ":extensions_resources",
@@ -90,10 +92,6 @@ source_set("test_support") {
     "//content/public/browser",
     "//extensions/common/api/cast_channel:cast_channel_proto",
   ]
-
-  if (is_win) {
-    cflags = [ "/wd4267" ]  # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
-  }
 }
 
 # GYP version: //extensions/extensions.gyp:extensions_shell_and_test_pak