Checkout mac_toolchain on Mac V8 builders

R=machenbach@chromium.org

Bug: chromium:863607
Change-Id: I999770d1dafbafe8bbe60b575a940f6feb90fae0
Reviewed-on: https://chromium-review.googlesource.com/1165551Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Sergiy Byelozyorov <sergiyb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54990}

DEPS

@@ -334,6 +334,13 @@ hooks = [
     'condition': 'checkout_win',
     'action': ['python', 'v8/build/vs_toolchain.py', 'update'],
   },
+  {
+    # Update the Mac toolchain if necessary.
+    'name': 'mac_toolchain',
+    'pattern': '.',
+    'condition': 'checkout_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