Use hermetic XCode on Dawn standalone builders.

Bug:
Change-Id: I25fc33c01e2148ed2626ff6d1999d414c8c8f04b
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/33100
Commit-Queue: Stephen White <senorblanco@chromium.org>
Auto-Submit: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Stephen White <senorblanco@chromium.org>
diff --git a/DEPS b/DEPS
index 1581d45..d93de16 100644
--- a/DEPS
+++ b/DEPS
@@ -136,6 +136,14 @@
                '--arch=x64'],
   },
   {
+    # Update the Mac toolchain if possible, this makes builders use "hermetic XCode" which is
+    # is more consistent (only changes when rolling build/) and is cached.
+    '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': '.',