Update: with help of a Chart.js contributor we've confirmed that it's a bug with Chart.js' way of loading things, see this comment and above for details. So below workaround is probably as good as it gets for now.

I ran into the same issue, and I made a bug report for it on the Chart.js repository. I also found a workaround, which is to add this to moment.min.js when building/packing your extension:

// Minified momentjs code on this line;
window.moment = moment;

In my case I use Powershell to dump files into a build folder:

Remove-Item build -Recurse -Force -Confirm:$false
mkdir build

Copy-Item node_modules/moment/min/moment.min.js build/
Copy-Item node_modules/chart.js/dist/Chart.js build/
Copy-Item src/js/app.js build/app.js
Copy-Item manifest.json build/manifest.json

# Workaround for:
# See also:
Add-Content -Path build/moment.min.js -Value "`n`nwindow.moment = moment;"

Not really a solution or answer, but useful enough to post I suppose...

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