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/codecs/multiplex/multiplex_decoder_adapter.cc

@@ -139,10 +139,10 @@ int32_t MultiplexDecoderAdapter::Decode(
   }
 
   if (image.component_count == 1) {
-    RTC_DCHECK(decoded_data_.find(input_image._timeStamp) ==
+    RTC_DCHECK(decoded_data_.find(input_image.Timestamp()) ==
                decoded_data_.end());
     decoded_data_.emplace(std::piecewise_construct,
-                          std::forward_as_tuple(input_image._timeStamp),
+                          std::forward_as_tuple(input_image.Timestamp()),
                           std::forward_as_tuple(kAXXStream));
   }
   int32_t rv = 0;