Promises: Add support for Finally

Finally is essentially a combiation of Then and Catch except its callback
doesn't take any arguments.

Design: https://docs.google.com/document/d/1l12PAJgEtlrqTXKiw6mk2cR2jP7FAfCCDr-DGIdiC9w/edit

Bug: 906125
Change-Id: I0f29c697cdf1525ad88869ccc1fe5a8bfa19a95f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1601038
Reviewed-by: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Fran├žois Doray <fdoray@chromium.org>
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662298}
7 files changed