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[Extensions] Enable promise support on the tabs 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: Iab033c7cd7174d82be4740bcf1ee4fca5288665f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2514807Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Tim Judkins <tjudkins@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826846}