…ltip (#34221)

This is intended to be used by various client side resources where the
transfer size is interesting to know how it'll perform in various
network conditions. Not intended to be added by the server.

For now it's only added internally by DevTools itself on img/css but
I'll add it from Flight Client too in a follow up.

This now shows this as the "transfer size" which is the encoded body
size + headers/overhead. Where as the "fileSize" that I add to images is
the decoded body size, like what you'd see on disk. This is what Chrome
shows so it's less confusing if you compare Network tab and this view.