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[Extensions] Enable promise support on the management API

This CL changes over the json schema to use the new returns_async format
to define callbacks, enabling promise support on those APIs.

Bug: 328932
Change-Id: I13caaf4f9eff77db7cb0afc1516ed485e1908556
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2515142Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Tim Judkins <tjudkins@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826848}