Web renderer breaks when dom.textMetrics.fontBoundingBox is disabled

[themmj]

Symptom:
A blank page after the initial loading screen with
Uncaught TypeError: CanvasRenderingContext2D.createLinearGradient: Argument 2 is not a finite floating-point value.
in the console.

Steps to reproduce
Using Waterfox as the browser, open https://one.livesplit.org/
Alternatively, disable dom.textMetrics.fontBoundingBox.enabled in about:config and open https://one.livesplit.org/

Root cause
CanvasFont's top and bottom attribute are determined by TextMetrics.fontBoundingBoxAscend/Descend.
This is an experimental feature that is disabled by default for some browsers (e.g. Waterfox). If they are disabled, the Javascript-Structure for TextMetrics does not contain fontBoundingBoxAscend/Descend properties.
As a result top and bottom are populated with NaNs. These values are eventually used to set the y-bounds on a CanvasLabel which will be passed to createLinearGradient.

Workaround
Set dom.textMetrics.fontBoundingBox.enabled in about:config to true.

Comments
The easiest solution would be to set top and bottom to 0 if they are NaN. Optionally, fall back on actualBoundingBoxAscend/Descend beforehand which seem to be enabled by default on a lot more browsers. The FIXME above these calls suggests that it shouldn't be done like that anyway. Let me know if I should proceed with the simple fix as a PR or if a more elaborate fix is preferred.