[webengine] Support NOTIFICATIONS permission type

iframes are explicitly forbidden from requesting the NOTIFICATIONS
permission (see chromestatus.com/feature/6451284559265792). Because of
this, we cannot delegate to the top-level origin for permission
queries. This behavior is similarly reflected in
PermissionUtil::GetCanonicalOrigin.

Bug: 415082165
Test: Manual
Change-Id: Ied9cf3035572b671d01ae8d01204b61288e3b50d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6536214
Reviewed-by: Zijie He <zijiehe@google.com>
Reviewed-by: David Song <wintermelons@google.com>
Commit-Queue: Christopher Whelan <chwhelan@google.com>
Cr-Commit-Position: refs/heads/main@{#1459601}