[SPARK-5768] [WEB UI] Fix for incorrect memory in Spark UI

Fix for incorrect memory in Spark UI as per SPARK-5768

Author: Joshi <rekhajoshm@gmail.com>
Author: Rekha Joshi <rekhajoshm@gmail.com>

Closes #6972 from rekhajoshm/SPARK-5768 and squashes the following commits:

b678a91 [Joshi] Fix for incorrect memory in Spark UI
2fe53d9 [Joshi] Fix for incorrect memory in Spark UI
eb823b8 [Joshi] SPARK-5768: Fix for incorrect memory in Spark UI
0be142d [Rekha Joshi] Merge pull request #3 from apache/master
106fd8e [Rekha Joshi] Merge pull request #2 from apache/master
e3677c9 [Rekha Joshi] Merge pull request #1 from apache/master