Add Timestamp accessor methods to the EncodedImage class.

Bug: webrtc:9378
Change-Id: I59bf14f631f92f0f4e05f60d4af25641a23a53f9
Reviewed-on: https://webrtc-review.googlesource.com/82100
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23734}

modules/video_coding/encoded_frame.h

@@ -65,10 +65,12 @@ class VCMEncodedFrame : protected EncodedImage {
    *   Get frame length
    */
   size_t Length() const { return _length; }
+
   /**
-   *   Get frame timestamp (90kHz)
+   *   Frame RTP timestamp (90kHz)
    */
-  uint32_t TimeStamp() const { return _timeStamp; }
+  using EncodedImage::Timestamp;
+  using EncodedImage::SetTimestamp;
   /**
    *   Get render time in milliseconds
    */