Add tree closing configs for chromium.mac builders.

The Gatekeeper config includes all builders under this master, except
for mac-osxbeta-rel. But actually this builder is under chromium.fyi, so
it wouldn't be included anyway.

Bug: 1094768
Change-Id: Ide1321def6f5702c68645a28c14c8da91fe698d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2287329Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Auto-Submit: Owen Rodley <orodley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#786599}

infra/config/lib/ci.star

@@ -833,6 +833,7 @@ def mac_builder(
       goma_backend = goma_backend,
       mastername = 'chromium.mac',
       os = os,
+      tree_closing = True,
       **kwargs
   )
 
@@ -938,6 +939,7 @@ def thin_tester(
       mastername = mastername,
       triggered_by = triggered_by,
       goma_backend = None,
+      tree_closing = True,
       **kwargs
   )