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}

video/video_stream_encoder_unittest.cc

@@ -656,7 +656,7 @@ class VideoStreamEncoderTest : public ::testing::Test {
         const RTPFragmentationHeader* fragmentation) override {
       rtc::CritScope lock(&crit_);
       EXPECT_TRUE(expect_frames_);
-      last_timestamp_ = encoded_image._timeStamp;
+      last_timestamp_ = encoded_image.Timestamp();
       last_width_ = encoded_image._encodedWidth;
       last_height_ = encoded_image._encodedHeight;
       encoded_frame_event_.Set();