Simplify RTPSender::RegisterRtpHeaderExtension DCHECK instead of run-time check extension type is correct.
Simplify RTPSender::RegisterRtpHeaderExtension
DCHECK instead of run-time check extension type is correct.

BUG=webrtc:1994

Review-Url: https://codereview.webrtc.org/2795723002
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#17524}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: fab482b32684b65f2ad9c6c7a967a4802dacff82

Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk