Reland "peerconnection: prefer spec names for signaling state"

This is a reland of f79bfc65e52a35d27cf0db2d212e94043fb44da3
the tests that have blocked the roll have been marked as allowed to fail.

Original change's description:
> peerconnection: prefer spec names for signaling state
>
> Map the internal state names to the spec ones defined in
>   https://w3c.github.io/webrtc-pc/#rtcsignalingstate-enum
> instead of exposing them. This only affects the (not specified)
> error strings.
>
> Bug: None
> Change-Id: Ib0b35bb3106b1688e8386f6fdd0b8c7fdebaf1dc
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/178390
> Reviewed-by: Henrik Boström <hbos@webrtc.org>
> Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
> Cr-Commit-Position: refs/heads/master@{#31591}

Bug: chromium:1101699
Change-Id: Ia21cec9e76fbaa4df2fa5a80409a7c80fedc4faa
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/178562
Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31914}
diff --git a/pc/peer_connection.cc b/pc/peer_connection.cc
index 541f04e..7cee2c7 100644
--- a/pc/peer_connection.cc
+++ b/pc/peer_connection.cc
@@ -251,21 +251,23 @@
   return cricket::CF_NONE;
 }
 
+// Map internal signaling state name to spec name:
+//  https://w3c.github.io/webrtc-pc/#rtcsignalingstate-enum
 std::string GetSignalingStateString(
     PeerConnectionInterface::SignalingState state) {
   switch (state) {
     case PeerConnectionInterface::kStable:
-      return "kStable";
+      return "stable";
     case PeerConnectionInterface::kHaveLocalOffer:
-      return "kHaveLocalOffer";
+      return "have-local-offer";
     case PeerConnectionInterface::kHaveLocalPrAnswer:
-      return "kHavePrAnswer";
+      return "have-local-pranswer";
     case PeerConnectionInterface::kHaveRemoteOffer:
-      return "kHaveRemoteOffer";
+      return "have-remote-offer";
     case PeerConnectionInterface::kHaveRemotePrAnswer:
-      return "kHaveRemotePrAnswer";
+      return "have-remote-pranswer";
     case PeerConnectionInterface::kClosed:
-      return "kClosed";
+      return "closed";
   }
   RTC_NOTREACHED();
   return "";