Provide a default implementation for PeerConnectionInterface::ice_state().

Default implementation is necessary to remove implementations in
downstream unit test mocks. Method will be entirely removed in upcomming
reland of https://codereview.webrtc.org/2256663002/ .

BUG=webrtc:6299

Review-Url: https://codereview.webrtc.org/2306833003
Cr-Commit-Position: refs/heads/master@{#14053}
diff --git a/webrtc/api/peerconnectioninterface.h b/webrtc/api/peerconnectioninterface.h
index bcb6170..8ba2460 100644
--- a/webrtc/api/peerconnectioninterface.h
+++ b/webrtc/api/peerconnectioninterface.h
@@ -507,7 +507,10 @@
   // TODO(bemasc): Remove ice_state when callers are changed to
   // IceConnection/GatheringState.
   // Returns the current IceState.
-  virtual IceState ice_state() = 0;
+  virtual IceState ice_state() {
+      RTC_NOTREACHED();
+    return kIceNew;
+  }
   virtual IceConnectionState ice_connection_state() = 0;
   virtual IceGatheringState ice_gathering_state() = 0;