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

Review URL: https://codereview.chromium.org/750263002

Cr-Commit-Position: refs/heads/master@{#305466}
2 files changed