Block "save link as" action if a URL is blocked for supervised users

The UI for "save link as" is disabled for websites that are manually blocked. This doesn't work well for sites blocked by the mature content
filter, which requires an async call. Therefore, the download is blocked
after the user has selected "save link as" to prevent supervised users
from downloading mature content.

Bug: b:259197739
Change-Id: I25c1ce11ef1a14001f0a0deac41f3d786a383d5d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4676071
Commit-Queue: Courtney Wong <courtneywong@chromium.org>
Reviewed-by: Aga Wronska <agawronska@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1169524}