Add implementation of EncodedFrame::Timestamp.

This is a preparation for inheriting the method from the base class,
and delete the corresponding redundant timestamp member.

TBR: magjed@webrtc.org
Bug: webrtc:9378
Change-Id: I27a0ec83fb20ac3da58ba32b86cf794a154deca0
Reviewed-on: https://webrtc-review.googlesource.com/83123
Commit-Queue: Niels Moller <nisse@webrtc.org>
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23594}

api/video/encoded_frame.cc

@@ -15,5 +15,7 @@ namespace video_coding {
 
 bool EncodedFrame::delayed_by_retransmission() const { return 0; }
 
+uint32_t EncodedFrame::Timestamp() const { return timestamp; }
+
 }  // namespace video_coding
 }  // namespace webrtc

api/video/encoded_frame.h

@@ -59,7 +59,7 @@ class EncodedFrame : public webrtc::VCMEncodedFrame {
   virtual bool GetBitstream(uint8_t* destination) const = 0;
 
   // The capture timestamp of this frame.
-  virtual uint32_t Timestamp() const = 0;
+  virtual uint32_t Timestamp() const;
 
   // When this frame was received.
   virtual int64_t ReceivedTime() const = 0;