Bugzilla

So that doesn't work.  I've attached patches that plumb everything, but they're actually not necessary to repro the problem, you can repro a variant of it with a clean tree (the second set of steps).

STR:
1. Install the [JetBrains font](https://fonts.google.com/specimen/JetBrains+Mono)
2. Go to https://ritter.vg/misc/ff/font-jetbrains.html and confirm all three are using different fonts.  (If they're not, you're going to need to reset prefs or reload the browser or reinstall the font or something)
3. Set `layout.css.font-visibility.trackingprotection` to 1 and probably restart the browser to be safe
4. Confirm that in regular browser window, you see all three different fonts, and in PBM the middle font matches the bottom.
5. Exempt the page from ETP, it reloads.  Same behavior.
6. Copy the URL, open a new tab, paste it, and you should see the middle font in JetBrains now.

That's a page _reload_.  Here's another way to reproduce this where a page should be restyled in-place without a reload:  

1. Do steps 1-4
5. Edit the pref `layout.css.font-visibility.trackingprotection` to 3 [which should cause UpdateFontVisbility() to be called and a restyle to happen](https://searchfox.org/mozilla-central/source/layout/base/nsPresContext.cpp#640).
6. Observe nothing changed

So that doesn't work.  I've attached patches that plumb everything, but they're actually not necessary to repro the problem, you can repro a variant of it with a clean tree (the second set of steps).

STR:
1. Apply patches
1. Install the [JetBrains font](https://fonts.google.com/specimen/JetBrains+Mono)
2. Go to https://ritter.vg/misc/ff/font-jetbrains.html and confirm all three are using different fonts.  (If they're not, you're going to need to reset prefs or reload the browser or reinstall the font or something)
3. Set `layout.css.font-visibility.trackingprotection` to 1 and probably restart the browser to be safe
4. Confirm that in regular browser window, you see all three different fonts, and in PBM the middle font matches the bottom.
5. Exempt the page from ETP, it reloads.  Same behavior.
6. Copy the URL, open a new tab, paste it, and you should see the middle font in JetBrains now.

That's a page _reload_.  Here's another way to reproduce this where a page should be restyled in-place without a reload, and this happens on a clean tree:  

1. Do steps 2-5 
5. Edit the pref `layout.css.font-visibility.trackingprotection` to 3 [which should cause UpdateFontVisbility() to be called and a restyle to happen](https://searchfox.org/mozilla-central/source/layout/base/nsPresContext.cpp#640).
6. Observe nothing changed