[SPARK-26109][WEBUI] Duration in the task summary metrics table and the task table are different

## What changes were proposed in this pull request?
Task summary table displays the summary of the task table in the stage page. However, the 'Duration' metrics of 'task summary' table and 'task table' are not matching. The reason is because, in the 'task summary' we display 'executorRunTime' as the duration, and in the 'task table' the actual duration of the task. Except duration metrics, all other metrics are properly displaying in the task summary.

In Spark2.2, used to show 'executorRunTime' as duration in the 'taskTable'. That is why, in summary metrics also the 'exeuctorRunTime' shows as the duration. So, we need to show 'executorRunTime' as the duration in the tasks table to follow the same behaviour as the previous versions of spark.

## How was this patch tested?
Before patch:

![screenshot from 2018-11-19 04-32-06](https://user-images.githubusercontent.com/23054875/48679263-1e4fff80-ebb4-11e8-9ed5-16d892039e01.png)

After patch:
![screenshot from 2018-11-19 04-37-39](https://user-images.githubusercontent.com/23054875/48679343-e39a9700-ebb4-11e8-8df9-9dc3a28d4bce.png)

Closes apache#23081 from shahidki31/duratinSummary.

Authored-by: Shahid <shahidki31@gmail.com>
Signed-off-by: Sean Owen <sean.owen@databricks.com>

Author: shahidki31

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

@@ -843,7 +843,7 @@ private[ui] class TaskPagedTable(
         </div>
       </td>
       <td>{UIUtils.formatDate(task.launchTime)}</td>
-      <td>{formatDuration(task.duration)}</td>
+      <td>{formatDuration(task.taskMetrics.map(_.executorRunTime))}</td>
       <td class={TaskDetailsClassNames.SCHEDULER_DELAY}>
         {UIUtils.formatDuration(AppStatusUtils.schedulerDelay(task))}
       </td>
@@ -996,7 +996,9 @@ private[ui] object ApiHelper {
     HEADER_EXECUTOR -> TaskIndexNames.EXECUTOR,
     HEADER_HOST -> TaskIndexNames.HOST,
     HEADER_LAUNCH_TIME -> TaskIndexNames.LAUNCH_TIME,
-    HEADER_DURATION -> TaskIndexNames.DURATION,
+    // SPARK-26109: Duration of task as executorRunTime to make it consistent with the
+    // aggregated tasks summary metrics table and the previous versions of Spark.
+    HEADER_DURATION -> TaskIndexNames.EXEC_RUN_TIME,
     HEADER_SCHEDULER_DELAY -> TaskIndexNames.SCHEDULER_DELAY,
     HEADER_DESER_TIME -> TaskIndexNames.DESER_TIME,
     HEADER_GC_TIME -> TaskIndexNames.GC_TIME,