Make ECDSA default for RTCPeerConnection

BUG=

Review URL: https://codereview.webrtc.org/1649533002

Cr-Commit-Position: refs/heads/master@{#11409}
diff --git a/webrtc/api/objc/RTCSessionDescription.mm b/webrtc/api/objc/RTCSessionDescription.mm
index 7ed0760..5066301 100644
--- a/webrtc/api/objc/RTCSessionDescription.mm
+++ b/webrtc/api/objc/RTCSessionDescription.mm
@@ -86,6 +86,7 @@
     return RTCSdpTypeAnswer;
   } else {
     RTC_NOTREACHED();
+    return RTCSdpTypeOffer;
   }
 }