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

webrtc/api/peerconnectioninterface_unittest.cc

 /*
  *  Copyright 2012 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 585 matching lines @@
     }
 
     std::unique_ptr<rtc::RTCCertificateGeneratorInterface> cert_generator;
     bool dtls;
     if (FindConstraint(constraints,
                        webrtc::MediaConstraintsInterface::kEnableDtlsSrtp,
                        &dtls,
                        nullptr) && dtls) {
       cert_generator.reset(new FakeRTCCertificateGenerator());
     }
+    config.rtcp_mux_policy = PeerConnectionInterface::kRtcpMuxPolicyNegotiate;

[honghaiz3, 2016/06/22 16:58:44]

Is there any reason to set kRtcpMuxPolicyNegotiate here?
Do we have tests to cover the new default rtcp_mux_policy?
Seems that most other methods call this method. 

[Zhi Huang, 2016/06/22 21:41:28]

Some unit tests rely on the assumption that the policy is negotiate. But I think
it make more sense to refactoring the test rather than setting the config since
we are changing to require anyway. I will fix it in next patch.

     pc_ = pc_factory_->CreatePeerConnection(
         config, constraints, std::move(port_allocator),
         std::move(cert_generator), &observer_);
     ASSERT_TRUE(pc_.get() != NULL);
     observer_.SetPeerConnectionInterface(pc_.get());
     EXPECT_EQ(PeerConnectionInterface::kStable, observer_.state_);
   }
 
   void CreatePeerConnectionExpectFail(const std::string& uri) {
     PeerConnectionInterface::RTCConfiguration config;
@@ skipping 2196 matching lines @@
   FakeConstraints updated_answer_c;
   answer_c.SetMandatoryReceiveAudio(false);
   answer_c.SetMandatoryReceiveVideo(false);
 
   cricket::MediaSessionOptions updated_answer_options;
   EXPECT_TRUE(
       ParseConstraintsForAnswer(&updated_answer_c, &updated_answer_options));
   EXPECT_TRUE(updated_answer_options.has_audio());
   EXPECT_TRUE(updated_answer_options.has_video());
 }