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}
diff --git a/video/video_send_stream_tests.cc b/video/video_send_stream_tests.cc
index 3baf13a..cb11ebc 100644
--- a/video/video_send_stream_tests.cc
+++ b/video/video_send_stream_tests.cc
@@ -3037,7 +3037,7 @@
 
       uint8_t buffer[16] = {0};
       EncodedImage encoded(buffer, sizeof(buffer), sizeof(buffer));
-      encoded._timeStamp = input_image.timestamp();
+      encoded.SetTimestamp(input_image.timestamp());
       encoded.capture_time_ms_ = input_image.render_time_ms();
 
       for (size_t i = 0; i < kNumStreams; ++i) {