Set mac_xcode_version in DEPS

Bug: chromium:1100266
Change-Id: I20df6d0d5bdd07aa69c4eecf68c1d2152ddc1bec
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2272567Reviewed-by: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68578}

DEPS

@@ -32,6 +32,8 @@ vars = {
   'download_jsfunfuzz': False,
   'check_v8_header_includes': False,
 
+  'mac_xcode_version': 'default',
+
   # GN CIPD package version.
   'gn_version': 'git_revision:7d7e8deea36d126397bda2cf924682504271f0e1',