Use StatsReport::Value::ToString instead of the value member variable direct.

This can be landed once the interface change has been made in WebRTC and rolled into Chromium (see here: https://webrtc-codereview.appspot.com/40139004/).

BUG=webrtc:2822

Review URL: https://codereview.chromium.org/875483002

Cr-Commit-Position: refs/heads/master@{#318694}

content/renderer/media/peer_connection_tracker.cc

@@ -217,7 +217,7 @@ static base::DictionaryValue* GetDictValueStats(
 
   for (const auto& v : report.values()) {
     values->AppendString(v->display_name());
-    values->AppendString(v->value);
+    values->AppendString(v->ToString());
   }
 
   return dict;

content/renderer/media/rtc_peer_connection_handler.cc

@@ -409,7 +409,7 @@ class StatsResponse : public webrtc::StatsObserver {
     for (const auto& value : report.values) {
       response->addStatistic(idx,
           blink::WebString::fromUTF8(value->display_name()),
-          blink::WebString::fromUTF8(value->value));
+          blink::WebString::fromUTF8(value->ToString()));
     }
   }