The function to call when the task executes. If it
returns a webdriver.promise.Promise, the flow will wait
for it to be resolved before starting the next task.
Resolves this promise with the given value. If the value is itself a
promise and not a reference to this deferred, this instance will wait for
it before resolving.
Represents the eventual value of a completed operation. Each promise may be
in one of three states: pending, resolved, or rejected. Each promise starts
in the pending state and may make a single transition to either a
fulfilled or rejected state, at which point the promise is considered
resolved.