Make the default value of rtcp-mux policy to required.

Change the default value of rtcp-mux policy in RTCConfiguration.
Refactor the peerconnectioninterface and webrtcsession unit tests.

BUG=webrtc:6030

Review-Url: https://codereview.webrtc.org/2043193003
Cr-Commit-Position: refs/heads/master@{#15217}
diff --git a/webrtc/api/peerconnectioninterface.h b/webrtc/api/peerconnectioninterface.h
index b304b14..e14b79c 100644
--- a/webrtc/api/peerconnectioninterface.h
+++ b/webrtc/api/peerconnectioninterface.h
@@ -295,7 +295,7 @@
     // at the same time.
     IceServers servers;
     BundlePolicy bundle_policy = kBundlePolicyBalanced;
-    RtcpMuxPolicy rtcp_mux_policy = kRtcpMuxPolicyNegotiate;
+    RtcpMuxPolicy rtcp_mux_policy = kRtcpMuxPolicyRequire;
     TcpCandidatePolicy tcp_candidate_policy = kTcpCandidatePolicyEnabled;
     CandidateNetworkPolicy candidate_network_policy =
         kCandidateNetworkPolicyAll;