Make the output_period_ms argument to StartRtcEventLog optional

Intended to ease transition to new log format.

Bug: webrtc:6463, webrtc:8111
Change-Id: Icadaedb6a6a7d31038a45ff5eb0b054528f00f2f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/135944
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27920}
diff --git a/api/peer_connection_interface.h b/api/peer_connection_interface.h
index 623b869..41a1c0f 100644
--- a/api/peer_connection_interface.h
+++ b/api/peer_connection_interface.h
@@ -1074,8 +1074,14 @@
   // |output| and passes it on to Call, which will take the ownership. If the
   // operation fails the output will be closed and deallocated. The event log
   // will send serialized events to the output object every |output_period_ms|.
+  // Applications using the event log should generally make their own trade-off
+  // regarding the output period. A long period is generally more efficient,
+  // with potential drawbacks being more bursty thread usage, and more events
+  // lost in case the application crashes. If the |output_period_ms| argument is
+  // omitted, webrtc selects a default deemed to be workable in most cases.
   virtual bool StartRtcEventLog(std::unique_ptr<RtcEventLogOutput> output,
                                 int64_t output_period_ms);
+  virtual bool StartRtcEventLog(std::unique_ptr<RtcEventLogOutput> output);
 
   // Stops logging the RtcEventLog.
   // TODO(ivoc): Make this pure virtual when Chrome is updated.