Update existing uses of /wd4267 to use the GN config

ash/BUILD.gn

Index: ash/BUILD.gn
diff --git a/ash/BUILD.gn b/ash/BUILD.gn
index f6f0eb9c262bed365b380875f6922b35462b2f63..ed34a59a430b65c4c8a16db12c665a0669fe1c1d 100644
--- a/ash/BUILD.gn
+++ b/ash/BUILD.gn
@@ -62,7 +62,7 @@ component("ash") {
     ]
 
     # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
-    cflags = [ "/wd4267" ]
+    configs += [ "//build/config/compiler:no_size_t_to_int_warning" ]
   }
 
   if (use_x11) {
@@ -314,9 +314,7 @@ test("ash_unittests") {
     ]
   }
 
-  if (is_win) {
-    cflags = [ "/wd4267" ]
-  }
+  configs += [ "//build/config/compiler:no_size_t_to_int_warning" ]
 
   if (!use_x11) {
     sources -= [ "host/ash_window_tree_host_x11_unittest.cc" ]