Fix chained-task-progress.

[inamiy]

This pull request will fix unwanted progress-passing from upstream task (parentTask) to downstream.

As-Is

let task3 = task1.then { ... return task2 }
task3.progress { ...
    // called when both task1 & task2's progress() is called
}

NOTE: This is not a good implementation because task1 and task2 may have different Progress type, thus can't mix together.

To-Be

let task3 = task1.then { ... return task2 }
task3.progress { ... 
    // should be called only when task2's progress() is called, just like fulfilled value & rejected errorInfo
}