Make RTCStatsReport::ToString() return JSON-parseable string.

webrtc/pc/rtcstats_integrationtest.cc

 /*
  *  Copyright 2016 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 318 matching lines @@
       }
     }
     if (!missing_stats.empty()) {
       verify_successful = false;
       for (const char* missing : missing_stats) {
         EXPECT_TRUE(false) << "Missing expected stats type: " << missing;
       }
     }
     EXPECT_TRUE(verify_successful) <<
         "One or more problems with the stats. This is the report:\n" <<
-        report_->ToString();
+        report_->ToJson();
   }
 
   bool VerifyRTCCertificateStats(
       const RTCCertificateStats& certificate) {
     RTCStatsVerifier verifier(report_, &certificate);
     verifier.TestMemberIsDefined(certificate.fingerprint);
     verifier.TestMemberIsDefined(certificate.fingerprint_algorithm);
     verifier.TestMemberIsDefined(certificate.base64_certificate);
     verifier.TestMemberIsOptionalIDReference(
         certificate.issuer_certificate_id, RTCCertificateStats::kType);
@@ skipping 319 matching lines @@
   caller_ = nullptr;
   // Any pending stats requests should have completed in the act of destroying
   // the peer connection.
   EXPECT_TRUE(stats_obtainer->report());
 }
 #endif  // HAVE_SCTP
 
 }  // namespace
 
 }  // namespace webrtc