Right now we slow down frame rate immediately after first contentful paint.
But fcp is often almost empty, some tiny bit of text or so.
So we could let some more paints first and then slow down more.
Based on raptor testing, especially Amazon's pageload like this too.

https://treeherder.mozilla.org/#/jobs?repo=try&revision=eb66f679d27c124804e904044c4c3a107c8f88cd

fcp is often almost blank paint, but the next couple paints have more content.
But in order to not regress page load time, reduce frame rate later during the page load.

https://hg.mozilla.org/mozilla-central/rev/224337be28e3

This brought many big improvements on Raptor tp6! \0/

== Change summary for alert #20137 (as of Wed, 27 Mar 2019 09:15:50 GMT) ==

Improvements:

34% raptor-tp6-yandex-firefox loadtime windows10-64-pgo-qr opt 259.88 -> 172.25
23% raptor-tp6-instagram-firefox windows10-64-pgo-qr opt 207.82 -> 160.59
22% raptor-tp6-instagram-firefox linux64-pgo-qr opt 215.37 -> 167.04
22% raptor-tp6-amazon-firefox loadtime windows10-64-pgo-qr opt 416.29 -> 326.46
20% raptor-tp6-amazon-firefox loadtime windows7-32 pgo 422.21 -> 338.00
19% raptor-tp6-amazon-firefox loadtime linux64 pgo 570.75 -> 464.00
18% raptor-tp6-amazon-firefox loadtime windows10-64 pgo 418.79 -> 342.50
16% raptor-tp6-instagram-firefox windows10-64 pgo 206.00 -> 173.02
15% raptor-tp6-amazon-firefox loadtime linux64-pgo-qr opt 569.29 -> 482.00
14% raptor-tp6-yandex-firefox windows10-64-pgo-qr opt 157.31 -> 135.17
13% raptor-tp6-youtube-firefox loadtime windows10-64-pgo-qr opt 664.81 -> 581.00
12% raptor-tp6-youtube-firefox loadtime linux64-pgo-qr opt 707.69 -> 623.71
9% raptor-tp6-yandex-firefox loadtime windows10-64 pgo 212.19 -> 192.54
7% raptor-tp6-amazon-firefox linux64 pgo 392.53 -> 365.40
5% raptor-tp6-amazon-firefox linux64-pgo-qr opt 416.54 -> 396.72

For up to date results, see: https://treeherder.mozilla.org/perf.html#/alerts?id=20137