Add RTCConfiguration getter and setter methods. The immediate plan is to move some flags into an embedded MediaConfig (https://codereview.webrtc.org/1818033002/), which will be possible after Chrome is updated to use these new setter methods.

BUG=webrtc:4906
R=hbos@google.com, hbos@webrtc.org, perkj@webrtc.org, pthatcher@webrtc.org

Review URL: https://codereview.webrtc.org/1836083003 .

Cr-Commit-Position: refs/heads/master@{#12177}
diff --git a/webrtc/api/peerconnectioninterface_unittest.cc b/webrtc/api/peerconnectioninterface_unittest.cc
index 2c044e2..51968e6 100644
--- a/webrtc/api/peerconnectioninterface_unittest.cc
+++ b/webrtc/api/peerconnectioninterface_unittest.cc
@@ -2552,7 +2552,7 @@
   PeerConnectionInterface::RTCConfiguration config;
   FakeConstraints constraints;
 
-  config.disable_prerenderer_smoothing = true;
+  config.set_prerenderer_smoothing(false);
   const cricket::MediaConfig& media_config =
       TestCreatePeerConnection(config, &constraints);