Stop setting mac_xcode_version in DEPS

mac_xcode_version no longer has an effect and we have been
requested to remove it.

Also add updating the mac toolchain.

Bug: chromium:1147839,tint:197
Change-Id: I120c1b4494154c701efcbe70063f35e4f1d373b2
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/33000
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
Auto-Submit: Ryan Harrison <rharrison@chromium.org>

DEPS

diff --git a/DEPS b/DEPS
index 36db4e8..2fa9ad9 100644
--- a/DEPS
+++ b/DEPS
@@ -1,16 +1,8 @@
 use_relative_paths = True
 
 gclient_gn_args_file = 'build/config/gclient_args.gni'
-gclient_gn_args = [
-  'mac_xcode_version',
-]
 
 vars = {
-  # This can be overridden, e.g. with custom_vars, to download a nonstandard
-  # Xcode version in build/mac_toolchain.py
-  # instead of downloading the prebuilt pinned revision.
-  'mac_xcode_version': 'default',
-
   'chromium_git':  'https://chromium.googlesource.com',
   'github': '/external/github.com',
 
@@ -111,6 +103,13 @@
                '--arch=x64'],
   },
   {
+    # Update the Mac toolchain if necessary.
+    'name': 'mac_toolchain',
+    'pattern': '.',
+    'condition': 'checkout_mac',
+    'action': ['python', 'build/mac_toolchain.py'],
+  },
+  {
     # Update the Windows toolchain if necessary. Must run before 'clang' below.
     'name': 'win_toolchain',
     'pattern': '.',