[Extensions] Use Promise based language in JSON schemas.

Updates JSON schemas in `chrome/common/extensions/api/` to use Promise
based language for the `returns_async.description` field. This matches
the Promise API signatures that are given priority in the documentation
where these methods are shown.

Bug: N/A
Change-Id: I49917ca0f513cdf5d61db512b084ad198b38ff15
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7300086
Commit-Queue: Oliver Dunk <oliverdunk@chromium.org>
Reviewed-by: Tim <tjudkins@chromium.org>
Owners-Override: Oliver Dunk <oliverdunk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1562367}
NOKEYCHECK=True
GitOrigin-RevId: 0d1975a1aeffa961d34a06dafa4bb60f5e5300a2