Publish rtc event log api and default factory for it in api/

Bug: webrtc:10206
Change-Id: I34194ddb6fd2b0a3d7c553fadc9ddc1ea9740da0
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/137500
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28023}

video/video_quality_test.cc

diff --git a/video/video_quality_test.cc b/video/video_quality_test.cc
index be29714..5415f96 100644
--- a/video/video_quality_test.cc
+++ b/video/video_quality_test.cc
@@ -1222,8 +1222,8 @@
                                       RtcEventLog::kImmediateOutput);
     RTC_DCHECK(event_log_started);
   } else {
-    send_event_log_ = RtcEventLog::CreateNull();
-    recv_event_log_ = RtcEventLog::CreateNull();
+    send_event_log_ = absl::make_unique<RtcEventLogNull>();
+    recv_event_log_ = absl::make_unique<RtcEventLogNull>();
   }
 
   task_queue_.SendTask([this, &params, &send_transport, &recv_transport]() {
@@ -1443,8 +1443,8 @@
                                       /*output_period_ms=*/5000);
     RTC_DCHECK(event_log_started);
   } else {
-    send_event_log_ = RtcEventLog::CreateNull();
-    recv_event_log_ = RtcEventLog::CreateNull();
+    send_event_log_ = absl::make_unique<RtcEventLogNull>();
+    recv_event_log_ = absl::make_unique<RtcEventLogNull>();
   }
 
   task_queue_.SendTask([&]() {