[Files SWA]: Enable directory change events

Use source_url() rather than extension_id() to register listeners with
FileWatcher. Extracts origin from source_url(). Uses the extracted
origin to convert file definition to entry definition. Changes
DispatchToExtension to BroadcastEvent to dispatch onDirectoryChanged
event.


Bug: 1184083
Change-Id: I4926189aa61cc07e5b37191d9cfb3cb6373b4d88
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2801772
Reviewed-by: Josh Simmons <simmonsjosh@google.com>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Alex Danilo <adanilo@chromium.org>
Commit-Queue: Bo Majewski <majewski@chromium.org>
Cr-Commit-Position: refs/heads/master@{#886654}