A/B testing rootSets should provide commit times as well as revisions

A/B testing rootSets should provide commit times as well as revisions
https://bugs.webkit.org/show_bug.cgi?id=145207

Reviewed by Andreas Kling.

Some continuous build systems need the commit time as well as the revision number / hash so provide one
in the root sets but maintain the backwards compatibility with buildbots that use revision number directly.

* public/include/build-requests-fetcher.php:
(BuildRequestsFetcher::fetch_roots_for_set): Made the revision info an associative array that contains
the revision number as well as the commit time.
* tools/sync-with-buildbot.py:
(schedule_request): Removed "replacement" which was a superfluous copy of "roots". Use "revision" values
when the JSON configuration refers to "root". This is necessary in buildbot instances that require WebKit
revision to be specified on its own field instead of it being a JSON that contains "revision" and "time".

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@184658 268f45cc-cd09-0410-ab3c-d52691b4dbfc