Fix implicit conversion of scoped_refptr<T> to T* in components/nacl

Fix implicit conversion of scoped_refptr<T> to T* in components/nacl

Since it's generally an anti-pattern to keep a pointer to a
MessageLoopProxy without ref'ing it, the message_loop_ member is now
a scoped_refptr<T>.

BUG=110610
Committed: https://crrev.com/df07d905b283b05c3461d55bff06142ec6b8a2c4
Cr-Commit-Position: refs/heads/master@{#305466}

[dcheng, 2014-11-22 05:43:31]

dcheng@chromium.org changed reviewers:
+ mseaborn@chromium.org

[Mark Seaborn, 2014-11-24 18:27:32]

LGTM

[dcheng, 2014-11-24 18:29:05]

The CQ bit was checked by dcheng@chromium.org

[commit-bot: I haz the power, 2014-11-24 18:30:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/750263002/1

[commit-bot: I haz the power, 2014-11-24 19:13:56]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-11-24 19:16:35]

Patchset 1 (id:??) landed as
https://crrev.com/df07d905b283b05c3461d55bff06142ec6b8a2c4
Cr-Commit-Position: refs/heads/master@{#305466}