[Extensions] Prevent duplicate event dispatches to a single context

When an extension registered multiple listeners for the same event with
overlapping filters, a single event matching those filters could be
dispatched multiple times to the same execution context, triggering all
matching listeners for *each* dispatch received.

This change introduces tracking of dispatched contexts within
`EventRouter::DispatchEventToActiveListener`. A set maintains which
contexts have already received the event during the current dispatch
cycle. This ensures the event is sent to a specific context only once.

Fixed: 40365717
Bug: 439995191
Change-Id: I352b786fccb15a4cc8558024c69cddd43e4be922
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6961849
Commit-Queue: Andrea Orru <andreaorru@chromium.org>
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1519596}