Add tracing for slow paths in new video API.
Add tracing for slow paths in new video API.

Allows tracking what actually takes time in SetRemoteDescription and
SetLocalDescription.

BUG=1788
R=tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/38809004

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#8202}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 50fe359eb614e1bbe41124b9c19263019da0395d

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