replace NtpTime->Clock with Clock->NtpTime dependency

BUG=None

Review-Url: https://codereview.webrtc.org/2393723004
Cr-Commit-Position: refs/heads/master@{#16519}

webrtc/system_wrappers/include/clock.h

diff --git a/webrtc/system_wrappers/include/clock.h b/webrtc/system_wrappers/include/clock.h
index a209770..5066844 100644
--- a/webrtc/system_wrappers/include/clock.h
+++ b/webrtc/system_wrappers/include/clock.h
@@ -13,6 +13,7 @@
 
 #include <memory>
 
+#include "webrtc/system_wrappers/include/ntp_time.h"
 #include "webrtc/system_wrappers/include/rw_lock_wrapper.h"
 #include "webrtc/typedefs.h"
 
@@ -43,8 +44,14 @@
   // Retrieve an NTP absolute timestamp in milliseconds.
   virtual int64_t CurrentNtpInMilliseconds() const = 0;
 
+  // TODO(danilchap): Make pure virtual once implemented in derived classed
+  // replacing CurrentNtp function.
+  virtual NtpTime CurrentNtpTime() const;
+
   // Converts an NTP timestamp to a millisecond timestamp.
-  static int64_t NtpToMs(uint32_t seconds, uint32_t fractions);
+  static int64_t NtpToMs(uint32_t seconds, uint32_t fractions) {
+    return NtpTime(seconds, fractions).ToMs();
+  }
 
   // Returns an instance of the real-time system clock implementation.
   static Clock* GetRealTimeClock();