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I'm against this since it removes the incentive for Ubuntu/Chromium to fix the underlying issue. See #5742 (comment)

Also note that you've added the fonts before Courier New and monospace, which will result in a change of behavior for some people.

removes the incentive for Ubuntu/Chromium to fix the underlying issue

I still think that a simple workaround is better than waiting for a complete solution for ages. Note that the original issue (#5742) is created for over an year. And I don't see any real progress in #5742 either.

added the fonts before Courier New and monospace

This is intentional as #5742 mentioned Courier New is very different from the first preference, Droid Sans Mono. It was even suggested to remove Courier New from the list.

Good points. Wouldn't removing Courier New be the better option then, since hard-coding a font is what caused #5742 in the first place?

I'm happy to remove Courier New since I'm no fan of it anyway. However, Courier New is also in the lists of default fonts for Windows and Mac, so removing it may also affect those in some sense. If any of you really care, it can be done in another PR.

Courier New makes sense on Windows, I don't think so about Mac. On Linux monospace makes the most sense IMHO. Would moving Courier New after monospace fix the display issue for you? I think that would be the safest change.

moving Courier New after monospace

For my computer, it looks exactly the same. Probably once Courier New is installed it became the default font for monospace...

Or maybe it's the bug mentioned here: #5742 (comment)
Can you try putting monospace in single quotes?

Ah, indeed, putting monospace in single quotes did make it use DejaVu Sans Mono which is the one I installed.

Nice :) I would then suggest

const DEFAULT_LINUX_FONT_FAMILY = '\'Droid Sans Mono\', \'monospace\', monospace, \'Droid Sans Fallback\'';

for the constant. What do you think?

Yes, it sounds better than adding extra fallbacks.
I've submitted #34947 to replace this PR.