GitLab

Some system health stories have been overriding RunPageInteractions method directly
without implementing memory dump triggering.

This fixes those stories by changing them to override _DidLoadStory. It also add
tests to avoid mistakes like these in the future.

Review-Url: https://codereview.chromium.org/2924163005
Cr-Commit-Position: refs/heads/master@{#477949}