Fix CPU overuse caused by WebRTC event logs not being batched

Use batches of 5 seconds instead of immediate log emission.
(An upcoming CL should make this configurable from JS.)

Bug: 910576
Change-Id: Ib2b804e1e8c7798b744548fb343776fa8a315a65
Reviewed-on: https://chromium-review.googlesource.com/c/1356800
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Elad Alon <eladalon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612663}

content/renderer/media/webrtc/rtc_peer_connection_handler.cc

@@ -1848,13 +1848,14 @@ void RTCPeerConnectionHandler::StartEventLog(IPC::PlatformFileForTransit file,
 
 void RTCPeerConnectionHandler::StartEventLog() {
   DCHECK(task_runner_->RunsTasksInCurrentSequence());
+  constexpr int64_t kOutputPeriodMs = 5000;
   // TODO(eladalon): StartRtcEventLog() return value is not useful; remove it
   // or find a way to be able to use it.
   // https://crbug.com/775415
   native_peer_connection_->StartRtcEventLog(
       std::make_unique<RtcEventLogOutputSinkProxy>(
           peer_connection_observer_.get()),
-      webrtc::RtcEventLog::kImmediateOutput);
+      kOutputPeriodMs);
 }
 
 void RTCPeerConnectionHandler::StopEventLog() {