[SPARK-3067] JobProgressPage could not show Fair Scheduler Pools section sometimes

JobProgressPage could not show Fair Scheduler Pools section sometimes.
SparkContext starts webui and then postEnvironmentUpdate. Sometimes JobProgressPage is accessed between webui starting and postEnvironmentUpdate, then the lazy val isFairScheduler will be false. The Fair Scheduler Pools section will not display any more.
Ok, I'll update this comment when I merge it. Thanks.