[remoting WebRTC] Specify Plan-B semantics for SDP.

Plan-B is deprecated, but some non-trivial work is needed to update to
Unified Plan. Specifying Plan-B explicitly will keep things working
when WebRTC changes the default to Unified Plan.

Bug: 903012
Change-Id: I3afc86c87f79af068f93af384cc20229293be5c5
Reviewed-on: https://chromium-review.googlesource.com/c/1327546Reviewed-by: Joe Downing <joedow@chromium.org>
Commit-Queue: Lambros Lambrou <lambroslambrou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607312}

remoting/protocol/webrtc_transport.cc

@@ -236,6 +236,12 @@ class WebrtcTransport::PeerConnectionWrapper
 
     rtc_config.media_config.video.periodic_alr_bandwidth_probing = true;
 
+    // Currently, the host only supports the deprecated "Plan B" semantics, so
+    // specify it here.
+    // TODO(crbug.com/903012): Change to kUnifiedPlan and make any necessary
+    // changes for this to work.
+    rtc_config.sdp_semantics = webrtc::SdpSemantics::kPlanB;
+
     peer_connection_ = peer_connection_factory_->CreatePeerConnection(
         rtc_config, std::move(port_allocator), nullptr, this);
   }