[SPARK-9187][WebUI] Timeline view may show negative value for running tasks

[carsonwang]

For running tasks, the executorRunTime metrics is 0 which causes negative executorComputingTime in the timeline. It also causes an incorrect SchedulerDelay time.

[srowen]

CC @kayousterhout

[carsonwang]

retest this please

[SparkQA]

Test build #37 has finished for PR 7526 at commit 7b17db2.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #37821 has finished for PR 7526 at commit 7b17db2.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[kayousterhout]

LGTM