[AriaNotify] Introduce FireAriaNotificationEvent

1. Introducing a new method `FireAriaNotificationEvent` to the base
   `BrowserAccessibilityManager`; this method takes the data of an ARIA
   notification, posted to a given `AXNode`, and it's in charge of
   firing the platform-specific event that handles it through an
   override in each platform accessibility manager.

2. Adding logic in `BrowserAccessibilityManager::FireGeneratedEvent` to
   iterate over the posted ARIA notifications of an `AXNode` and make
   the respective calls to `FireAriaNotificationEvent`.

Bug: 326277796
Change-Id: Ib12c0837e16b26309b9d5157f629133293e40f9f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5403489


Commit-Queue: Ethan Jimenez <ethavar@microsoft.com>
Reviewed-by: Alison Maher <almaher@microsoft.com>
Reviewed-by: Benjamin Beaudry <benjamin.beaudry@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1279991}