Reimplement rtc::ToString and rtc::FromString without streams.

Bug: webrtc:8982
Change-Id: I3977435b035fdebef449732301d6e77fc899e7ba
Reviewed-on: https://webrtc-review.googlesource.com/86941
Commit-Queue: Jonas Olsson <jonasolsson@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24319}

examples/peerconnection/server/utils.cc

@@ -17,9 +17,9 @@
 using rtc::ToString;
 
 std::string int2str(int i) {
-  return ToString<int>(i);
+  return ToString(i);
 }
 
 std::string size_t2str(size_t i) {
-  return ToString<size_t>(i);
+  return ToString(i);
 }