Destroy WebRTCStatsReportCallbackResolver on main thread, fixes crash.

content/renderer/media/rtc_peer_connection_handler.cc

Index: content/renderer/media/rtc_peer_connection_handler.cc
diff --git a/content/renderer/media/rtc_peer_connection_handler.cc b/content/renderer/media/rtc_peer_connection_handler.cc
index 3240a7a2fbe2e9e97bb3f0bade3ce22d159fb618..98b5dbe1824e33270c1b2bc936aedb48d7d4f5b7 100644
--- a/content/renderer/media/rtc_peer_connection_handler.cc
+++ b/content/renderer/media/rtc_peer_connection_handler.cc
@@ -738,14 +738,6 @@ class GetRTCStatsCallback : public webrtc::RTCStatsCollectorCallback {
             this, report));
   }
 
-  void OnStatsDeliveredOnMainThread(
-      const rtc::scoped_refptr<const webrtc::RTCStatsReport>& report) {
-    DCHECK(main_thread_->BelongsToCurrentThread());
-    DCHECK(report);
-    callback_->OnStatsDelivered(std::unique_ptr<blink::WebRTCStatsReport>(
-        new RTCStatsReport(make_scoped_refptr(report.get()))));
-  }
-
  protected:
   GetRTCStatsCallback(
       const scoped_refptr<base::SingleThreadTaskRunner>& main_thread,
@@ -753,6 +745,18 @@ class GetRTCStatsCallback : public webrtc::RTCStatsCollectorCallback {
       : main_thread_(main_thread),
         callback_(callback) {
   }
+  ~GetRTCStatsCallback() override { DCHECK(!callback_); }
+
+  void OnStatsDeliveredOnMainThread(
+      const rtc::scoped_refptr<const webrtc::RTCStatsReport>& report) {
+    DCHECK(main_thread_->BelongsToCurrentThread());
+    DCHECK(report);
+    DCHECK(callback_);
+    callback_->OnStatsDelivered(std::unique_ptr<blink::WebRTCStatsReport>(
+        new RTCStatsReport(make_scoped_refptr(report.get()))));
+    // Make sure the callback is destroyed in the main thread as well.
+    callback_.reset();
+  }
 
   const scoped_refptr<base::SingleThreadTaskRunner> main_thread_;
   std::unique_ptr<blink::WebRTCStatsReportCallback> callback_;