Enable LUCI-Notify tree closing for chromium.

This will be submitted shortly after sending a PSA to infra-announc@

Bug: 1094768
Change-Id: I277dade09e7a0299a50b7b776dbd0279cd1f7a4b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2294503Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Owen Rodley <orodley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#787645}

infra/config/generated/luci-notify.cfg

@@ -4923,3 +4923,4 @@ notifiers {
     failed_step_regexp: "bot_update|compile|gclient runhooks|runhooks|update"
   }
 }
+tree_closing_enabled: true

infra/config/main.star

@@ -16,8 +16,7 @@ lucicfg.check_version(
 # Enable LUCI Realms support.
 lucicfg.enable_experiment('crbug.com/1085650')
 
-# Enable tree closing. Note that for now it is only in a "dry run" mode, where
-# it doesn't actually close the tree, just logs what it would have done.
+# Enable tree closing.
 lucicfg.enable_experiment("crbug.com/1054172")
 
 # Tell lucicfg what files it is allowed to touch
@@ -87,6 +86,10 @@ luci.milo(
     logo = 'https://storage.googleapis.com/chrome-infra-public/logo/chromium.svg',
 )
 
+luci.notify(
+    tree_closing_enabled = True,
+)
+
 exec('//recipes.star')
 
 exec('//notifiers.star')