Add crostini OWNERS for UI.

Same owners as the UI backend.

Test: None
Bug: None
Change-Id: I9b6e186334deeb4851db21afeead459108989c1a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2011588
Auto-Submit: David Munro <davidmunro@google.com>
Reviewed-by: Trent Apted <tapted@chromium.org>
Reviewed-by: Nicholas Verne <nverne@chromium.org>
Commit-Queue: Nicholas Verne <nverne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733837}

chrome/browser/ui/webui/chromeos/crostini_installer/OWNERS

+file://chrome/browser/chromeos/guest_os/OWNERS
+
 per-file *.mojom=set noparent
 per-file *.mojom=file://ipc/SECURITY_OWNERS
+
+# COMPONENT: UI>Shell>Containers

chrome/browser/ui/webui/chromeos/crostini_upgrader/OWNERS

+file://chrome/browser/chromeos/guest_os/OWNERS
+
 per-file *.mojom=set noparent
 per-file *.mojom=file://ipc/SECURITY_OWNERS
+
+# COMPONENT: UI>Shell>Containers