Support multiple URLs in PeerConnectionInterface::IceServer

This adds support for multiple URLs in a IceServer configuration as
defined in http://w3c.github.io/webrtc-pc/#idl-def-RTCIceServer.

BUG=2096
R=pthatcher@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/57489004

Cr-Commit-Position: refs/heads/master@{#9320}

talk/app/webrtc/peerconnectioninterface.h

@@ -175,7 +175,9 @@ class PeerConnectionInterface : public rtc::RefCountInterface {
   };
 
   struct IceServer {
+    // TODO(jbauch): Remove uri when all code using it has switched to urls.
     std::string uri;
+    std::vector<std::string> urls;
     std::string username;
     std::string password;
   };