Make class of static functions in rtp_to_ntp.h:

- UpdateRtcpList
- RtpToNtp

class RtpToNtpEstimator
- UpdateMeasurements
- Estimate

List with rtcp measurements is now private.

BUG=none

Review-Url: https://codereview.webrtc.org/2574133003
Cr-Commit-Position: refs/heads/master@{#15762}

webrtc/modules/rtp_rtcp/include/remote_ntp_time_estimator.h

@@ -14,7 +14,7 @@
 #include <memory>
 
 #include "webrtc/base/constructormagic.h"
-#include "webrtc/system_wrappers/include/rtp_to_ntp.h"
+#include "webrtc/system_wrappers/include/rtp_to_ntp_estimator.h"
 
 namespace webrtc {
 
@@ -43,7 +43,7 @@ class RemoteNtpTimeEstimator {
  private:
   Clock* clock_;
   std::unique_ptr<TimestampExtrapolator> ts_extrapolator_;
-  RtcpMeasurements rtcp_list_;
+  RtpToNtpEstimator rtp_to_ntp_;
   int64_t last_timing_log_ms_;
   RTC_DISALLOW_COPY_AND_ASSIGN(RemoteNtpTimeEstimator);
 };