Adding error enum to be used by PeerConnectionInterface methods.

webrtc/api/peerconnectioninterface_unittest.cc

Index: webrtc/api/peerconnectioninterface_unittest.cc
diff --git a/webrtc/api/peerconnectioninterface_unittest.cc b/webrtc/api/peerconnectioninterface_unittest.cc
index d673b4174a93a7a46aa2771477073b4c0b22ffe1..cfba3e67ec73cff287e139985b8a95ddb2250241 100644
--- a/webrtc/api/peerconnectioninterface_unittest.cc
+++ b/webrtc/api/peerconnectioninterface_unittest.cc
@@ -9,6 +9,7 @@
  */
 
 #include <memory>
+#include <sstream>
 #include <string>
 #include <utility>
 
@@ -2989,3 +2990,11 @@ TEST(CreateSessionOptionsTest, MediaConstraintsInAnswer) {
   EXPECT_TRUE(updated_answer_options.has_audio());
   EXPECT_TRUE(updated_answer_options.has_video());
 }
+
+TEST(RtcErrorTest, OstreamOperator) {
+  std::ostringstream oss;
+  oss << webrtc::RtcError::NONE << ' '
+      << webrtc::RtcError::INVALID_PARAMETER << ' '
+      << webrtc::RtcError::INTERNAL_ERROR;
+  EXPECT_EQ("NONE INVALID_PARAMETER INTERNAL_ERROR", oss.str());
+}