Add accessor methods for RTP timestamp of EncodedImage.

Intention is to make the member private, but downstream callers
must be updated to use the accessor methods first.

Bug: webrtc:9378
Change-Id: I3495bd8d545b7234fbea10abfd14f082caa420b6
Reviewed-on: https://webrtc-review.googlesource.com/82160
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24352}

modules/video_coding/encoded_frame.cc

@@ -35,8 +35,8 @@ void VCMEncodedFrame::Free() {
 }
 
 void VCMEncodedFrame::Reset() {
+  SetTimestamp(0);
   _renderTimeMs = -1;
-  _timeStamp = 0;
   _payloadType = 0;
   _frameType = kVideoFrameDelta;
   _encodedWidth = 0;