[WebPush] Add feature flag for change event on resubscription

This CL adds a feature flag for gating firing the
pushsubscriptionchange event upon manual resubscription to
notifications from a website. Since the pushsubscriptionchange event
is already implemented, but firing upon revocation, behind another
feature flag, this requires some custom code to conditionally enable
the common code path if either feature flag is enabled.

Bug: 407523313
Change-Id: Id39dec17807c8531add895cff96de45a98d855df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6421816
Reviewed-by: Peter Beverloo <peter@chromium.org>
Commit-Queue: Antonio Sartori <antoniosartori@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1445896}