Instead of playing the game of "how to measure transparency/contrast", why not draw some pixels (Chrome UI) around or near the icon that can't be controlled by the extension author? So it doesn't matter what the icon is, even if it's completely transparent? Could be as simple as a black border? Or some kind of tasteful drop-shadow or button effect?

That would avoid the challenge of detecting visibility, but it has some downsides:

* Just drawing a border says "there's something here", but not what it is; things can still look broken/unintuitive.  Luma inversion or using a default extension icon make it much clearer that "this is an extension".
* Something like luma inversion could help non-malicious cases, too: if the user has installed a custom theme and the extension's icon is not visible over that theme's toolbar color, this can "fix" the problem automatically.
* None of the rest of Chrome's toolbar uses such effects, and in general icon buttons in Chrome don't have borders, so either we'd have to live with extensions looking like a Win 95 effect dropped into a Win 10 world, or we'd need to redesign our UI language; neither course is very appealing.

Sorry Peter, didn't see this before I filed issue 805600.  Merging into that one to continue the discussion, but I'm very interested in your thoughts there.