[SPARK-17352][WEBUI] Executor computing time can be negative-number because 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 <sarutak@oss.nttdata.co.jp>

Closes #14908 from sarutak/SPARK-17352.