This change makes the code a little cleaner and reduces the number of
places we call PR_GetCurrentThread, which is important for Quantum DOM
scheduling work.
The conversion was largely automatic, via:
find netwerk/ -name \*.cpp | \
xargs sed -i -e 's/MOZ_ASSERT(PR_GetCurrentThread() == gSocketThread[^;]*/MOZ_ASSERT(OnSocketThread(), "not on socket thread")/'
and related invocations, with a few manual tweaks at the end.
