Set the tree status via milestone vars.

This keeps the scope of changes in a new milestone directory to the
vars.star file.

Bug: 922150
Change-Id: I82a951a019f24d2795343b41f8c8785852f1e2cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2098855Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>
Auto-Submit: Garrett Beaty <gbeaty@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749346}

infra/config/versioned/trunk/buckets/try.star

@@ -36,7 +36,7 @@ luci.cq_group(
     name = vars.cq_group.get(),
     cancel_stale_tryjobs = True,
     retry_config = cq.RETRY_ALL_FAILURES,
-    tree_status_host = 'chromium-status.appspot.com/',
+    tree_status_host = getattr(milestone_vars, 'tree_status_host', None),
     watch = cq.refset(
         repo = 'https://chromium.googlesource.com/chromium/src',
         refs = [milestone_vars.cq_ref_regexp],

infra/config/versioned/trunk/vars.star

 vars = struct(
     ref = 'refs/heads/master',
     cq_ref_regexp = 'refs/heads/.+',
+    # Delete this line for branches
+    tree_status_host = 'chromium-status.appspot.com/',
 )