Enable -Wno-language-extension-token except on Windows

It is required on Windows to be able to use __uuidof().

Bug: dawn:394
Change-Id: I02c1a4d36688c5985d5eb7f65737f6b821655ddc
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/20703
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Stephen White <senorblanco@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>

src/common/BUILD.gn

@@ -112,10 +112,14 @@ config("dawn_internal") {
       cflags += [ "-pedantic" ]
     }
 
+    # On Windows allow the use of __uuidof()
+    if (is_win) {
+      cflags += [ "-Wno-language-extension-token" ]
+    }
+
     # Flags for -pedantic warnings that haven't been fixed yet.
     cflags += [
       "-Wno-gnu-zero-variadic-macro-arguments",
-      "-Wno-language-extension-token",
       "-Wno-microsoft-enum-value",
     ]
   }