Add Component: to OWNERS for private_membership and shell-encryption

Noticed these were missing when doing binary size review.

Bug: None
Change-Id: I8177a8e2138b58a68c036300a89fa2e4073f11a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2484046
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Amr Aboelkher <amraboelkher@chromium.org>
Commit-Queue: Amr Aboelkher <amraboelkher@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819787}

third_party/private_membership/OWNERS

 amraboelkher@chromium.org
 kwlyeo@chromium.org
 
+# COMPONENT: Enterprise>CloudPolicy

third_party/shell-encryption/OWNERS

 amraboelkher@chromium.org
 kwlyeo@chromium.org
+
+# COMPONENT: Enterprise>CloudPolicy