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/utility/ivf_file_writer_unittest.cc

@@ -50,7 +50,7 @@ class IvfFileWriterTest : public ::testing::Test {
       if (use_capture_tims_ms) {
         frame.capture_time_ms_ = i;
       } else {
-        frame._timeStamp = i;
+        frame.SetTimestamp(i);
       }
       if (!file_writer_->WriteFrame(frame, codec_type))
         return false;