GetDefaultLocalAddress should return the bestIP
on an IPv6 network that contains the actual default local address. This is for preventing potential IP leaking.

BUG=webrtc:5376

Review URL: https://codereview.webrtc.org/1837823005

Cr-Commit-Position: refs/heads/master@{#12417}
diff --git a/webrtc/base/network.h b/webrtc/base/network.h
index ee22d5e..7c328ff 100644
--- a/webrtc/base/network.h
+++ b/webrtc/base/network.h
@@ -160,6 +160,8 @@
  private:
   friend class NetworkTest;
 
+  Network* GetNetworkFromAddress(const rtc::IPAddress& ip) const;
+
   EnumerationPermission enumeration_permission_;
 
   NetworkList networks_;