[Taskcluster] add a trigger branch for WebKitGTK (#19512)

Following up on web-platform-tests/wpt#19466.

Author: foolip

.taskcluster.yml

@@ -19,8 +19,9 @@ tasks:
                 event.ref == "refs/heads/triggers/chrome_dev": [{name: chrome, channel: dev}],
                 event.ref == "refs/heads/triggers/firefox_stable": [{name: firefox, channel: stable}],
                 event.ref == "refs/heads/triggers/firefox_beta": [{name: firefox, channel: beta}],
-                event.ref == "refs/heads/triggers/firefox_nightly": [{name: firefox, channel: nightly}]
-                }
+                event.ref == "refs/heads/triggers/firefox_nightly": [{name: firefox, channel: nightly}],
+                event.ref == "refs/heads/triggers/webkitgtk_minibrowser_stable": [{name: webkitgtk_minibrowser, channel: stable}]
+              }
           each(browser):
             $map:
               - [testharness, 1, 15]