Add Browser Performance widget display condition

What does this MR do?

Provide some clarification why the Browser Performance widget is not display sometimes. Specifically with regards to the widget not displaying if the last run pipeline against the widget does not contain the performance test with the artifacts.

Related issues

https://gitlab.zendesk.com/agent/tickets/447435

This was tested on GitLab.com on a project that uses merge trains using the below gitlab-ci.yml config:

include:
  template: Verify/Browser-Performance.gitlab-ci.yml

workflow:
  rules:
    - if: $CI_PIPELINE_SOURCE == 'merge_request_event'
    - if: $CI_COMMIT_TAG
    - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH

test_job:
  stage:
    build
  script:
    echo "Test Job"

browser_performance:
  stage:
    test
  variables:
    URL: https://gitlab.com
  rules:
    - if: '$CI_MERGE_REQUEST_EVENT_TYPE == "merge_train"'
      when: never
    - when: on_success

The browser performance job run successfully on the merge results pipeline and the widget displays in the merge request UI but once the merge-train pipeline runs and the job is not included the widget disappears from the MR.

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