Convert Garden-o-matic network code to Promises

This is part of a larger refactoring to simplify (and hopefully speed up) garden-o-matic.

BUG=
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/164373016

git-svn-id: svn://svn.chromium.org/blink/trunk@167442 bbb929c8-8fbe-4397-9dbb-9b2b20218538