PeerConnection: stop including api/proxy.h indirectly.

This change allows WebRTC to use TRACE_EVENT directly in proxy.h.

Bug: webrtc:12787
Change-Id: I59a23447d69e7a3b63e05d33490e31e876361959
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2910123


Commit-Queue: Markus Handell <handellm@google.com>
Reviewed-by: Tommi <tommi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#886232}