Use rtc::ToString instead of std::to_string in `SocketAddress::Po…

…rtAsString()` (#156)

Justification for this change is that `std::to_string` should be avoided
as it uses the user's locale and calls to it get serialized, which is
bad for concurrency.

My actual motivation for this is quite bizarre. Before this change, with
Zed's use of the LiveKit Rust SDK, I was getting connection strings that
were not valid utf-8, instead having a port of
`3\u0000\u0000\u001c\u0000`. I have not figured out how that could
happen or why this change fixes it.

rtc_base/socket_address.cc

@@ -162,7 +162,7 @@ std::string SocketAddress::HostAsSensitiveURIString() const {
 }
 
 std::string SocketAddress::PortAsString() const {
-  return std::to_string(port_);
+  return rtc::ToString(port_);
 }
 
 std::string SocketAddress::ToString() const {