[SPARK-17352][WEBUI] Executor computing time can be negative-number b…

…ecause of calculation error

## What changes were proposed in this pull request?

In StagePage, executor-computing-time is calculated but calculation error can occur potentially because it's calculated by subtraction of floating numbers.

Following capture is an example.

<img width="949" alt="capture-timeline" src="https://cloud.githubusercontent.com/assets/4736016/18152359/43f07a28-7030-11e6-8cbd-8e73bf4c4c67.png">

## How was this patch tested?

Manual tests.

Author: Kousuke Saruta <[email protected]>

Closes #14908 from sarutak/SPARK-17352.

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

@@ -651,9 +651,9 @@ private[ui] class StagePage(parent: StagesTab) extends WebUIPage("stage") {
         }
         val executorComputingTime = executorRunTime - shuffleReadTime - shuffleWriteTime
         val executorComputingTimeProportion =
-          (100 - schedulerDelayProportion - shuffleReadTimeProportion -
+          math.max(100 - schedulerDelayProportion - shuffleReadTimeProportion -
             shuffleWriteTimeProportion - serializationTimeProportion -
-            deserializationTimeProportion - gettingResultTimeProportion)
+            deserializationTimeProportion - gettingResultTimeProportion, 0)
 
         val schedulerDelayProportionPos = 0
         val deserializationTimeProportionPos =