It would be great if you can create a test website (maybe something on GitHub Pages or on
http://jsbin.com/) which relies on default font size and reproduces the problem when loaded in WebView. I don't think you need to provide a full sample app, just the repro URL will be fine for now.
If you file the bug, then someone on my team can figure out when the change was introduced and whether this effect was intentional or not. There are several thousands of code changes between 130 and 131, but we can narrow the range if we can reproduce the problem.