Add OWNER file to ash/bloom

This folder is maintained by the Assistive team.

Bug: N/A
Change-Id: Ide7af49046a8326985acd19ab715e885961b673d
Tests: N/A
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2490841
Commit-Queue: Jeroen Dhollander <jeroendh@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Auto-Submit: Jeroen Dhollander <jeroendh@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819516}

ash/bloom/OWNERS

+file://chromeos/assistant/OWNERS
+
+# COMPONENT: UI>Shell>Assistant