Replace supervised_user_error_page OWNERS

Add yilkal and michaelpg, and remove previous owners.

Bug: None
Change-Id: Ib06fab3d5992b6454f6f7ec49b89b08e040dd108
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1974757Reviewed-by: Yilkal Abe <yilkal@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728296}

chrome/browser/supervised_user/supervised_user_error_page/OWNERS

-pam@chromium.org
-treib@chromium.org
+michaelpg@chromium.org
+yilkal@chromium.org
+
+# Previous owner, feel free to add for any questions or historical context:
+# treib@chromium.org
 
 # COMPONENT: Services>SupervisedUser