Bug: Error in displaying the top 3 worst stages in a job execution

**Problem**
There is an error in displaying the top 3 worst stages in a job execution. This information is showed to user in the performance graph at job history page when the filter is set to be heuristics.

**Problem Demonstration**
Lets suppose we have a look at execution of job with job definition id as 101103 on 25th March, 2018.

Here clearly we can see that stage 3, stage 2 and stage 6 are claimed to worst stages of this execution. But from the table it is clearly visible that for stage 6, all heuristics are green and have severity level as NONE. Also for stage 4, 3 of the heuristics have severity level as Moderate. Surely there is a bug.

After doing the code fix I got the following result,

Start at the job history page's performance graph with the heuristics filter and trace how the top three worst stages are selected and displayed. Compare the selected stages with the heuristic severity table using the example execution, and verify that the graph lists the stages with the greatest severities.

Written by the indexing model from the issue text.