Whitelist a new dir for Chrome sign-in screen extensions policy cache

The special directory will be added that will be used by Chrome for
storing the cache of the component policies for the extensions
installed in the sign-in profile.

This is needed for:
https://chromium-review.googlesource.com/#/c/406831/

BUG=chromium:644304
TEST=manual

Change-Id: I0f78321edbb7056f0dd7282a813207d4771e2268
Reviewed-on: https://chromium-review.googlesource.com/408141
Commit-Ready: Maksim Ivanov <emaxx@chromium.org>
Tested-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
diff --git a/client/site_tests/security_StatefulPermissions/security_StatefulPermissions.py b/client/site_tests/security_StatefulPermissions/security_StatefulPermissions.py
index 72efc47..9c05cd7 100644
--- a/client/site_tests/security_StatefulPermissions/security_StatefulPermissions.py
+++ b/client/site_tests/security_StatefulPermissions/security_StatefulPermissions.py
@@ -37,6 +37,7 @@
                                  "/encrypted/var/cache/echo",
                                  "/encrypted/var/cache/external_cache",
                                  "/encrypted/var/cache/shared_extensions",
+                                 "/encrypted/var/cache/signin_profile_component_policy",
                                  "/encrypted/var/cache/touch_trial/selection",
                                  "/encrypted/var/lib/cromo",
                                  "/encrypted/var/lib/opencryptoki",