Return both IPv6 and IPv4 address from the lookup.

webrtc/base/nethelpers.cc

Index: webrtc/base/nethelpers.cc
diff --git a/webrtc/base/nethelpers.cc b/webrtc/base/nethelpers.cc
index fddfdaf58b5832318fe4f73d4f2ad7262f5b03bf..1f6bb8df7b7f71628524df9bd1fa27240ab9fe7b 100644
--- a/webrtc/base/nethelpers.cc
+++ b/webrtc/base/nethelpers.cc
@@ -44,8 +44,24 @@ int ResolveHostname(const std::string& hostname, int family,
   addresses->clear();
   struct addrinfo* result = NULL;
   struct addrinfo hints = {0};
-  // TODO(djw): For now this is IPv4 only so existing users remain unaffected.
-  hints.ai_family = AF_INET;
+  hints.ai_family = family;
+  // |family| here will almost always be AF_UNSPEC, because |family| comes from
+  // AsyncResolver::addr_.family(), which comes from a SocketAddress constructed
+  // with a hostname. When a SocketAddress is constructed with a hostname, its
+  // family is AF_UNSPEC. However, if someday in the future we construct
+  // a SocketAddress with both a hostname and a family other than AF_UNSPEC,
+  // then it would be possible to get a specific family value here.
+
+  // The behavior of AF_UNSPEC is roughly "get both ipv4 and ipv6", as
+  // documented by the various operating systems:
+  // Linux: http://man7.org/linux/man-pages/man3/getaddrinfo.3.html
+  // Windows: https://msdn.microsoft.com/en-us/library/windows/desktop/
+  // ms738520(v=vs.85).aspx
+  // Mac: https://developer.apple.com/legacy/library/documentation/Darwin/
+  // Reference/ManPages/man3/getaddrinfo.3.html
+  // Android (source code, not documentation):
+  // https://android.googlesource.com/platform/bionic/+/
+  // 7e0bfb511e85834d7c6cb9631206b62f82701d60/libc/netbsd/net/getaddrinfo.c#1657
   hints.ai_flags = AI_ADDRCONFIG;
   int ret = getaddrinfo(hostname.c_str(), NULL, &hints, &result);
   if (ret != 0) {