GitLab

Rather than passing a separate callback into each of the methods that
create a Transaction, have the completed signal be a method on the
|Platform|.

BUG=1002262

Change-Id: Ie63e5ace91177127efa9f84ef49c4412139f13ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2500276
Commit-Queue: Adam Langley <agl@chromium.org>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#822764}