Add a flag to filter out high-cost networks.

webrtc/p2p/client/basicportallocator.cc

Index: webrtc/p2p/client/basicportallocator.cc
diff --git a/webrtc/p2p/client/basicportallocator.cc b/webrtc/p2p/client/basicportallocator.cc
index 14f7b13212db920adbfd7bc957eb1894ef798e8d..20fc84a38a22fc05eeed1942a1231a1ba94aaf27 100644
--- a/webrtc/p2p/client/basicportallocator.cc
+++ b/webrtc/p2p/client/basicportallocator.cc
@@ -417,6 +417,19 @@ void BasicPortAllocatorSession::GetNetworks(
                                           network->type();
                                  }),
                   networks->end());
+
+  if (flags() & PORTALLOCATOR_DISABLE_COSTLY_NETWORKS) {
+    uint16_t lowest_cost = rtc::kNetworkCostMax;
+    for (rtc::Network* network : *networks) {
+      lowest_cost = std::min<uint16_t>(lowest_cost, network->GetCost());
+    }
+    networks->erase(std::remove_if(networks->begin(), networks->end(),
+                                   [lowest_cost](rtc::Network* network) {
+                                     return network->GetCost() >
+                                            lowest_cost + rtc::kNetworkCostLow;
+                                   }),
+                    networks->end());
+  }
 }
 
 // For each network, see if we have a sequence that covers it already.  If not,