Download the selected version of hermetic Xcode via custom_vars.

Taken from crrev.com/782205.

Bug: pdfium:1590
Change-Id: Ibb954bc6dcf9f7fc8111b728e64aaa226f5f1ca7
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/73632
Reviewed-by: Daniel Hosseinian <dhoss@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/DEPS b/DEPS
index a5500f5..aa0527f 100644
--- a/DEPS
+++ b/DEPS
@@ -270,7 +270,8 @@
     'name': 'mac_toolchain',
     'pattern': '.',
     'condition': 'checkout_mac',
-    'action': ['python', 'build/mac_toolchain.py'],
+    'action': ['python', 'build/mac_toolchain.py',
+               '--xcode-version', Var('mac_xcode_version')],
   },
   # Pull clang-format binaries using checked-in hashes.
   {