Make RtcEventLogOutputSink GC-ed by Oilpan

This way we are avoid raw pointers referencing to it
(see rtc_peer_connection_handler.cc|h).

BUG=787254
R=guidou@chromium.org

Change-Id: I0b3375336c6051d6ac21776335ae325a849a3d7c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1944370
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#722466}

third_party/blink/renderer/modules/peerconnection/rtc_peer_connection_handler.cc

@@ -834,7 +834,9 @@ class RTCPeerConnectionHandler::WebRtcSetDescriptionObserverImpl
 class RTCPeerConnectionHandler::Observer
     : public GarbageCollected<RTCPeerConnectionHandler::Observer>,
       public PeerConnectionObserver,
-      public blink::RtcEventLogOutputSink {
+      public RtcEventLogOutputSink {
+  USING_GARBAGE_COLLECTED_MIXIN(Observer);
+
  public:
   Observer(const base::WeakPtr<RTCPeerConnectionHandler>& handler,
            scoped_refptr<base::SingleThreadTaskRunner> task_runner)
@@ -854,7 +856,7 @@ class RTCPeerConnectionHandler::Observer
     }
   }
 
-  void Trace(Visitor* visitor) {}
+  void Trace(Visitor* visitor) override {}
 
  protected:
   // TODO(hbos): Remove once no longer mandatory to implement.

third_party/blink/renderer/platform/peerconnection/rtc_event_log_output_sink.h

@@ -5,11 +5,12 @@
 #ifndef THIRD_PARTY_BLINK_RENDERER_PLATFORM_PEERCONNECTION_RTC_EVENT_LOG_OUTPUT_SINK_H_
 #define THIRD_PARTY_BLINK_RENDERER_PLATFORM_PEERCONNECTION_RTC_EVENT_LOG_OUTPUT_SINK_H_
 
+#include "third_party/blink/renderer/platform/heap/garbage_collected.h"
 #include "third_party/blink/renderer/platform/platform_export.h"
 
 namespace blink {
 
-class PLATFORM_EXPORT RtcEventLogOutputSink {
+class PLATFORM_EXPORT RtcEventLogOutputSink : public GarbageCollectedMixin {
  public:
   virtual ~RtcEventLogOutputSink() = default;
 

third_party/blink/renderer/platform/peerconnection/rtc_event_log_output_sink_proxy.h

@@ -7,6 +7,7 @@
 
 #include <memory>
 
+#include "third_party/blink/renderer/platform/heap/persistent.h"
 #include "third_party/blink/renderer/platform/platform_export.h"
 #include "third_party/webrtc/api/rtc_event_log_output.h"
 
@@ -29,7 +30,7 @@ class PLATFORM_EXPORT RtcEventLogOutputSinkProxy final
   bool Write(const std::string& output) override;
 
  private:
-  RtcEventLogOutputSink* const sink_;
+  CrossThreadWeakPersistent<RtcEventLogOutputSink> sink_;
 };
 
 }  // namespace blink