Fix chromium-style warnings.

Separate the null implementation from rtp_rtcp_defines.h, and follow chromium style guide for virtual functions.

BUG=webrtc:163

Review-Url: https://codereview.webrtc.org/2400993002
Cr-Original-Commit-Position: refs/heads/master@{#14738}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 509eadd554de6bf938da08071c5d2c2541703134
11 files changed