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-Original-Commit-Position: refs/heads/master@{#15217}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 4dfb8cef514bf030f08d61289543e32127366f12

api/test/testsdpstrings.h

diff --git a/api/test/testsdpstrings.h b/api/test/testsdpstrings.h
index 158b16c..b203aad 100644
--- a/api/test/testsdpstrings.h
+++ b/api/test/testsdpstrings.h
@@ -29,6 +29,7 @@
     "c=IN IP4 74.95.2.170\r\n"
     "a=rtpmap:109 opus/48000/2\r\n"
     "a=ptime:20\r\n"
+    "a=rtcp-mux\r\n"
     "a=rtpmap:0 PCMU/8000\r\n"
     "a=rtpmap:8 PCMA/8000\r\n"
     "a=rtpmap:101 telephone-event/8000\r\n"
@@ -46,6 +47,7 @@
     " 10.0.254.2 rport 50962\r\n"
     "m=video 38826 RTP/SAVPF 120\r\n"
     "c=IN IP4 74.95.2.170\r\n"
+    "a=rtcp-mux\r\n"
     "a=rtpmap:120 VP8/90000\r\n"
     "a=sendrecv\r\n"
     "a=candidate:0 1 UDP 2112946431 172.16.191.1 62017 typ host\r\n"