Add a flag to filter out high-cost networks.

webrtc/api/peerconnection.cc

Index: webrtc/api/peerconnection.cc
diff --git a/webrtc/api/peerconnection.cc b/webrtc/api/peerconnection.cc
index dda0eebb074ce5d03db6258c3388963027c3d2ca..7e7484e89e2715e40f1364c21866b3ae9af445c9 100644
--- a/webrtc/api/peerconnection.cc
+++ b/webrtc/api/peerconnection.cc
@@ -2115,6 +2115,11 @@ bool PeerConnection::InitializePortAllocator_w(
     LOG(LS_INFO) << "TCP candidates are disabled.";
   }
 
+  if (configuration.gather_low_cost_network_only) {

[pthatcher1, 2016/05/17 20:44:27]

It seems like we should make this an enum policy like tcp_candidate_policy. 
Perhaps candidate_network_policy == kCandidateNetworkPolicyLowCost (default
would be kCandidateNetworkPolicyAll

[honghaiz3, 2016/05/18 05:55:02]

Done.

+    portallocator_flags |= cricket::PORTALLOCATOR_GATHER_LOW_COST_NETWORK_ONLY;
+    LOG(LS_INFO) << "Only gather on lower-cost networks";
+  }
+
   port_allocator_->set_flags(portallocator_flags);
   // No step delay is used while allocating ports.
   port_allocator_->set_step_delay(cricket::kMinimumStepDelay);