Reland "Rename SIGNALING and WORKER to PRIMARY and SECONDARY"

This is a reland of a37f2bd9421868e222d591d3371486a6ab939fd6

Original change's description:
> Rename SIGNALING and WORKER to PRIMARY and SECONDARY
>
> This makes the proxy macros less confusing when the secondary thread
> is sometimes the worker thread, sometimes the networking thread.
>
> Bug: none
> Change-Id: I1a8cebb82d09be44fe40e80c861bcfb47b9928e8
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/208763
> Reviewed-by: Tommi <tommi@webrtc.org>
> Commit-Queue: Harald Alvestrand <hta@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#33346}

Bug: none
Change-Id: If46a6679ac0fc947797dd7be87626ef7702faca2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/208845
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33349}
diff --git a/api/rtp_receiver_interface.h b/api/rtp_receiver_interface.h
index 8984d71..e0ace54 100644
--- a/api/rtp_receiver_interface.h
+++ b/api/rtp_receiver_interface.h
@@ -120,8 +120,8 @@
 // Define proxy for RtpReceiverInterface.
 // TODO(deadbeef): Move this to .cc file and out of api/. What threads methods
 // are called on is an implementation detail.
-BEGIN_SIGNALING_PROXY_MAP(RtpReceiver)
-PROXY_SIGNALING_THREAD_DESTRUCTOR()
+BEGIN_PRIMARY_PROXY_MAP(RtpReceiver)
+PROXY_PRIMARY_THREAD_DESTRUCTOR()
 PROXY_CONSTMETHOD0(rtc::scoped_refptr<MediaStreamTrackInterface>, track)
 PROXY_CONSTMETHOD0(rtc::scoped_refptr<DtlsTransportInterface>, dtls_transport)
 PROXY_CONSTMETHOD0(std::vector<std::string>, stream_ids)