Track connected subframes per-Document instead of per-Node.

Track connected subframes per-Document instead of per-Node.

This frees up 10 bits of space in NodeRareData for other uses.

BUG=none

[dcheng, 2016-03-04 23:20:57]

dcheng@chromium.org changed reviewers:
+ esprehn@chromium.org, ojan@chromium.org

[dcheng, 2016-03-04 23:26:20]

Here's my entry for overly clever CL of the week.

It gets rid of the connected subframe count in NodeRareData… but it's not clear
to me that doing so is immediately valuable: do we actually need those bits atm?
NodeRareData's size isn't going to change, and the resulting patch is somewhat
difficult to understand.

There's also a weird use-after-poison I'm trying to debug, but I'm relatively
confident that I can figure that out if we think this approach is worth
pursuing.

[esprehn, 2016-03-07 22:01:16]

Yeah we could do this to save the bits if we really need them, it's not clear to
me we do need them today though. I'm not sure this is much less complexity
either.