Add ToString() methods to classes with << operators, preparing for deprecations.

Bug: webrtc:8982
Change-Id: I9b8792a229539dd9848f4d9936fe343f4bf9ad49
Reviewed-on: https://webrtc-review.googlesource.com/63200
Commit-Queue: Jonas Olsson <jonasolsson@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22705}
diff --git a/api/rtcerror.cc b/api/rtcerror.cc
index f9a31d0..5a8ba03 100644
--- a/api/rtcerror.cc
+++ b/api/rtcerror.cc
@@ -94,8 +94,13 @@
 }
 
 std::ostream& operator<<(std::ostream& stream, RTCErrorType error) {
+  return stream << ToString(error);
+}
+
+// TODO(jonasolsson): Change to use absl::string_view when it's available.
+std::string ToString(RTCErrorType error) {
   int index = static_cast<int>(error);
-  return stream << kRTCErrorTypeNames[index];
+  return std::string(kRTCErrorTypeNames[index]);
 }
 
 }  // namespace webrtc