Encode log events periodically instead of for every event.

Updated unit test to take output_period and random seed as parameters.
Updated the peerconnection interface to allow passing in an output_period.

This is in preparation of some upcoming CLs that will change the format
to store batches of delta-encoded values.


Bug: webrtc:8111
Change-Id: Id5d9844dfad8d8edad346cd7cbebc7eadaaa5416
Reviewed-on: https://webrtc-review.googlesource.com/22600
Commit-Queue: Björn Terelius <terelius@webrtc.org>
Reviewed-by: Elad Alon <eladalon@webrtc.org>
Reviewed-by: Tommi <tommi@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20736}

api/peerconnectioninterface.h

@@ -828,10 +828,19 @@ class PeerConnectionInterface : public rtc::RefCountInterface {
   // Start RtcEventLog using an existing output-sink. Takes ownership of
   // |output| and passes it on to Call, which will take the ownership. If the
   // operation fails the output will be closed and deallocated.
+  // TODO(eladalon): Deprecate and remove this.
   virtual bool StartRtcEventLog(std::unique_ptr<RtcEventLogOutput> output) {
     return false;
   }
 
+  // Start RtcEventLog using an existing output-sink. Takes ownership of
+  // |output| and passes it on to Call, which will take the ownership. If the
+  // operation fails the output will be closed and deallocated.
+  virtual bool StartRtcEventLog(std::unique_ptr<RtcEventLogOutput> output,
+                                int64_t output_period_ms) {
+    return false;
+  }
+
   // Stops logging the RtcEventLog.
   // TODO(ivoc): Make this pure virtual when Chrome is updated.
   virtual void StopRtcEventLog() {}