Refactor rtc_unittests into several targets.

Also fix some warnings.

NOTRY=True
BUG=webrtc:6954

Review-Url: https://codereview.webrtc.org/2611663002
Cr-Original-Commit-Position: refs/heads/master@{#15900}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: da8dcfb43e88719652ebe7c84107015a932b6ce5
14 files changed