Add a flag to filter out high-cost networks.

webrtc/api/peerconnection.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 2097 matching lines @@
   } else if (webrtc::field_trial::FindFullName("WebRTC-IPv6Default") ==
              "Disabled") {
     portallocator_flags &= ~(cricket::PORTALLOCATOR_ENABLE_IPV6);
   }
 
   if (configuration.tcp_candidate_policy == kTcpCandidatePolicyDisabled) {
     portallocator_flags |= cricket::PORTALLOCATOR_DISABLE_TCP;
     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);
   port_allocator_->set_candidate_filter(
       ConvertIceTransportTypeToCandidateFilter(configuration.type));
 
   // Call this last since it may create pooled allocator sessions using the
   // properties set above.
   port_allocator_->SetConfiguration(stun_servers, turn_servers,
                                     configuration.ice_candidate_pool_size);
@@ skipping 10 matching lines @@
   port_allocator_->set_candidate_filter(
       ConvertIceTransportTypeToCandidateFilter(configuration.type));
   // Call this last since it may create pooled allocator sessions using the
   // candidate filter set above.
   port_allocator_->SetConfiguration(stun_servers, turn_servers,
                                     configuration.ice_candidate_pool_size);
   return true;
 }
 
 }  // namespace webrtc