Rename page.id -> page._loader_id and propagate the change throughout. This was
my attempt at pretending that page.id (and page._frame_id) weren't CDP-sepcific.
But they are, and it's a lot cleaner to treat them this way. Might seem
unnecessary, but without this, after page -> frame, you'd end up with:
frame.id
frame._frame_id
Which is weird? What is `frame.id` if it isn't the frame id and if that's the
case, what's frame_id? Now it'll be:
frame._loader_id
frame._frame_id
Which removes the ambiguity, makes the CDP code a bit more obvious, and doesn't
try to hide the fact that these are CDP things that, for now at least, pollute
the code a little.
842affd83b