This adds a transform which will evaluate the 'serviceworker-e10s' key. If
False, the task is returned as is. If True, the task is run with the
'dom.serviceWorkers.parent_intercept' pref enabled. If 'both' the task is split
into two tasks, one with the pref, and one without.
Differential Revision: https://phabricator.services.mozilla.com/D7480