In bug 1369662, I altered the tabpaint test to compute a more precise delta from "tab requested" to "tab presented". Part of the way I did that was by using the fetchStart PerformanceTiming value. Even though fetchStart == navigationStart when nothing else has been loaded (which is the case in the tabpaint test), mstange pointed out that this subtlety might be confusing for new folks hacking on the test, and that using navigationStart might be clearer. This bug is to do that. : https://developer.mozilla.org/en-US/docs/Web/API/PerformanceTiming
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