[SPARK-9829] [WEBUI] Display the update value for peak execution memory

The peak execution memory is not correct because it shows the sum of finished tasks' values when a task finishes.

This PR fixes it by using the update value rather than the accumulator value.

Author: zsxwing <zsxwing@gmail.com>

Closes #8121 from zsxwing/SPARK-9829.

core/src/main/scala/org/apache/spark/ui/jobs/StagePage.scala

@@ -860,7 +860,7 @@ private[ui] class TaskDataSource(
     }
     val peakExecutionMemoryUsed = taskInternalAccumulables
       .find { acc => acc.name == InternalAccumulator.PEAK_EXECUTION_MEMORY }
-      .map { acc => acc.value.toLong }
+      .map { acc => acc.update.getOrElse("0").toLong }
       .getOrElse(0L)
 
     val maybeInput = metrics.flatMap(_.inputMetrics)