cleanup RTCPSender
rtc::scoped_ptr -> std::unique_ptr.
CrticalSectionWrapper -> CriticalSection.
assert -> DCHECK.
removed unused headers.
removed unused using.
removed unused member field.

BUG=webrtc:5520, webrtc:5565
R=åsapersson

Review URL: https://codereview.webrtc.org/1806603002

Cr-Original-Commit-Position: refs/heads/master@{#12091}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 56036ffc45765a2fd6c849a87835b475cb608d9b