Get rid of the manual destructor in AudioCodingModuleImpl

By converting three raw pointers to scoped_ptrs, we can eliminate the
need for a manually-defined destructor, and generally sleep better at
night.

R=henrik.lundin@webrtc.org

Review URL: https://codereview.webrtc.org/1310213003 .

Cr-Commit-Position: refs/heads/master@{#9776}
3 files changed