[build] Update mac toolchain via deps

TBR=tmrts@chromium.org

Bug: chromium:986701
Change-Id: I86cf6c73dfdb4c76eaad530bd22da385a834655b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1714653Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62931}

DEPS

@@ -410,6 +410,13 @@ hooks = [
     'condition': 'checkout_win',
     'action': ['python', 'v8/build/vs_toolchain.py', 'update'],
   },
+  {
+    # Update the Mac toolchain if necessary.
+    'name': 'mac_toolchain',
+    'pattern': '.',
+    'condition': 'checkout_ios or checkout_mac',
+    'action': ['python', 'v8/build/mac_toolchain.py'],
+  },
   # Pull binutils for linux, enabled debug fission for faster linking /
   # debugging when used with clang on Ubuntu Precise.
   # https://code.google.com/p/chromium/issues/detail?id=352046