Ever since we moved to 100% async JS, mwLoadStart stopped being useful, since it races with other things (causing the variation in described in the original description of this task).
References to `mediaWikiLoadStart` variable:
* [x] mediawiki/extensions/ImageMetrics: uses `mediaWikiLoadStart` as offset. – https://gerrit.wikimedia.org/r/387510/
* [ ] mediawiki/extensions/NavigationTiming: used to compute the `mediaWikiLoadComplete` delta. – Use fetchStart instead?
* [ ] mediawiki/extensions/VisualEditor: uses `mediaWikiLoadStart` in a `Math.max()` – Safe to remove?
* [ ] puppet: navtiming.py – Safe to remove
* [ ] puppet: coal.py – Safe to remove
* [ ] mediawiki/core - definition
References to `mwLoadStart` mark:
* [ ] mediawiki/core - definition. Safe to remove. Maybe rename to mwStartup?