I've executed some page load tests on Alexa's top 500 sites to evaluate the current progress tracker behavior.

I've found that the signal responsible for capturing the page resource download progress is too noisy to provide any significance.

Let's simplify the progress tracker by removing this signal.
Before further tweaking the progress tracker mechanics, I would like to monitor our telemetry probes (bug 1499418) to assess the impact of this change in the wild.

M3 because meta bug 1454477 is M3. This progress tracking change will affect our perceived page load performance and should show up in our Nimbledroid tests.

A note on the expected effect on our telemetry and Nimbledroid results: I expect this change to have a neutral to minimally positive effect on progress bar performance.

This step should support further signal refinements by reducing the complexity of the progress tracker signal handling and by removing a noisy signal to increase correlation between the perceived and the reported page load progress.

P1 because we want this for Fenix MVP.

https://hg.mozilla.org/mozilla-central/rev/b6b1d95ca780