VoE: format VoEBase according to new style guide

Purely cosmetic changes:
1. virtual => override
2. NULL => nullptr
3. data member name: underscore prefix => suffix
4. clang format

R=kwiberg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/49669004

Cr-Commit-Position: refs/heads/master@{#8997}
2 files changed