I don't know if FrameId is related to an <iframe>, and whether each Page has
1 implicit "frame". But, playwright seems to treat frameId and targetId as
interchangeable, and chrome seems to agree (at leas to some degree); chrome will
return a targetId and reuse that value for the frameId.
So the simplest solution is just to remove our concept of a frameId and use
targetId exclusively. This doesn't seem to cause any issues with puppeteer.