1 second delay during first draw on this profile, too much canvas work

Profile: https://share.firefox.dev/4ejNTKZ

Steps to reproduce:
1. Load https://share.firefox.dev/4mWUCNa
2. Watch the page during load

Expected results:
The timeline should have a reasonable height from the start.

Actual results:
The timeline is rendered with a big height, then 1 second passes, and then the page corrects itself.

During the 1-second pause, the CanvasRenderer thread in the GPU process is busy doing software canvas rendering.
See https://share.firefox.dev/4ejNTKZ (and see the screenshot track for the bad initial rendering).

Maybe the memory track is drawn at the wrong size initially?

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-1274)