Mark shell-encryption and private_membership code as security critical.

Per third_party/README.chromium.template, the "Security Critical" entry
should say "yes" if the code is shipped in releases. Given
third_party/shell-encryption code generates static initializers, as seen
in crrev.com/c/2430846, the code is definitely shipped. Presumably
private_membership code is in the same situation.

Bug: 1066937
Change-Id: Id5a64d0fba01ace9f21f35d75662face7087d358
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2469838
Reviewed-by: Amr Aboelkher <amraboelkher@google.com>
Reviewed-by: Amr Aboelkher <amraboelkher@chromium.org>
Commit-Queue: Amr Aboelkher <amraboelkher@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816938}
GitOrigin-RevId: 4c630ad9995e1e107acba727377fa2e4e15ebfd4
diff --git a/README.chromium b/README.chromium
index 7a9bb73..caf9903 100644
--- a/README.chromium
+++ b/README.chromium
@@ -3,7 +3,7 @@
 Version: f94f58852e9b3edaf2766e348a69a70f596bb9dd
 License: Apache Version 2.0
 License File: src/LICENSE
-Security Critical: no
+Security Critical: yes
 
 Description: This project is a library for fully-homomorphic symmetric-key
 encryption. It uses Ring Learning with Errors (RLWE)-based encryption to make it