disable nacl by default
[OSX] run through libuv after done work in Blink

And check uv_backend_timeout for idle handles of libuv
cherry pick 2 commits from 0.8 branch:
b8a046390a5641bce0bf91b96e5cd2a75f4d2851
6237f932b937befa23b26ac4664a3c1959a9a77e