Add explicit copy constructors to RTPHeader and RTPHeaderExtension

Explicit copy-constructors are required by chromium. (No copy constructors were used until now, but a different CL requires them.)

BUG=None
TBR=stefan@webrtc.org

Review-Url: https://codereview.webrtc.org/3006343002
Cr-Original-Commit-Position: refs/heads/master@{#19777}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 98b1b7d59e5b9eab61211d3b13dab87cb3bb18ce