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

BUG=chromium:653087

Review-Url: https://codereview.webrtc.org/2983243002
Cr-Commit-Position: refs/heads/master@{#19180}
diff --git a/webrtc/pc/rtcstats_integrationtest.cc b/webrtc/pc/rtcstats_integrationtest.cc
index 47fa0e3..56a4ef1 100644
--- a/webrtc/pc/rtcstats_integrationtest.cc
+++ b/webrtc/pc/rtcstats_integrationtest.cc
@@ -336,7 +336,7 @@
     }
     EXPECT_TRUE(verify_successful) <<
         "One or more problems with the stats. This is the report:\n" <<
-        report_->ToString();
+        report_->ToJson();
   }
 
   bool VerifyRTCCertificateStats(