Only enable -Wmax-tokens when the Mac build is using the default Xcode version

The token limits are set after the default version (used on the CQ) and may not
be appropriate for the headers in other versions.

Bug: 1049569
Change-Id: I6230efe9d7e56459fbae0355751bf81e6e5c5dfc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2315691Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791583}

build/config/compiler/BUILD.gn

@@ -140,7 +140,8 @@ declare_args() {
   # so only enable them by default for mainstream build configs.
   enable_wmax_tokens =
       !is_official_build &&
-      ((is_mac && target_cpu == "x64") ||
+      ((is_mac && target_cpu == "x64" && !use_system_xcode &&
+        mac_xcode_version == "default") ||
        (is_linux && !is_chromeos && target_cpu == "x64") ||
        (is_win && target_cpu == "x86") || (is_win && target_cpu == "x64") ||
        (is_android && target_cpu == "arm") ||