Only run build/mac_toolchain.py on Mac.

Use the "condition" mechanism in gclient runhooks to this, instead of
assuming build/mac_toolchain.py will take care of OS detection. Changes
upstream, like https://crrev.com/784530, may break that assumption.

Change-Id: Ie68f9ad48365006304f86c1c53c78ab8cfba4a94
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/72013
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Daniel Hosseinian <dhoss@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

DEPS

diff --git a/DEPS b/DEPS
index 8a5d710..4e39f98 100644
--- a/DEPS
+++ b/DEPS
@@ -269,6 +269,7 @@
     # Update the Mac toolchain if necessary.
     'name': 'mac_toolchain',
     'pattern': '.',
+    'condition': 'checkout_mac',
     'action': ['python', 'build/mac_toolchain.py'],
   },
   # Pull clang-format binaries using checked-in hashes.