[SPARK-24704][WEBUI] Fix the order of stages in the DAG graph

## What changes were proposed in this pull request?

Before:

![wx20180630-155537](https://user-images.githubusercontent.com/1438757/42123357-2c2e2d84-7c83-11e8-8abd-1c2860f38783.png)

After:

![wx20180630-155604](https://user-images.githubusercontent.com/1438757/42123359-32fae990-7c83-11e8-8a7b-cdcee94f9123.png)

## How was this patch tested?

Manual tests.

Author: Stan Zhai <mail@stanzhai.site>

Closes #21680 from stanzhai/fix-dag-graph.

core/src/main/scala/org/apache/spark/status/AppStatusStore.scala

@@ -471,7 +471,7 @@ private[spark] class AppStatusStore(
 
   def operationGraphForJob(jobId: Int): Seq[RDDOperationGraph] = {
     val job = store.read(classOf[JobDataWrapper], jobId)
-    val stages = job.info.stageIds
+    val stages = job.info.stageIds.sorted
 
     stages.map { id =>
       val g = store.read(classOf[RDDOperationGraphWrapper], id).toRDDOperationGraph()