Eliminate separate boolean tracking socket writeability in

Eliminate separate boolean tracking socket writeability in
QuicConnection; query the writer instead. This CL tries to minimize
behavior changes, so no actual bugfixes. Connection will now check
writer's status instead of its local flag, so it will refuse to
write sooner.

Merge internal change: 59603011

R=rch@chromium.org

[Ryan Hamilton, 2014-01-15 05:24:20]

lgtm