Reland "Help app: Add flags to correctly show content for allowed features."

This is a reland of commit 28d81e70

Original change's description:
> Help app: Add flags to correctly show content for allowed features.
>
> Bug: b/321865996
> Change-Id: I8a3bfa4582b019c5bc0bcee92795b6b9e69930a8
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5401299


> Reviewed-by: Zufeng Wang <zufeng@google.com>
> Commit-Queue: Callistus Tan <callistus@google.com>
> Cr-Commit-Position: refs/heads/main@{#1279498}

Bug: b/321865996
Change-Id: I85fabb5dceb17a7c432a4f2db695feb9da7202d1
Cq-Include-Trybots: luci.chrome.try:chromeos-betty-pi-arc-chrome,linux-chromeos-chrome;luci.chromium.try:linux-chromeos-rel
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5404544


Commit-Queue: Callistus Tan <callistus@google.com>
Reviewed-by: Zufeng Wang <zufeng@google.com>
Cr-Commit-Position: refs/heads/main@{#1281534}