Logic for finding frame references moved from PacketBuffer to new class
RtpFrameReferenceFinder.

BUG=webrtc:5514

Review-Url: https://codereview.webrtc.org/1961053002
Cr-Commit-Position: refs/heads/master@{#12725}
diff --git a/webrtc/modules/video_coding/video_coding.gypi b/webrtc/modules/video_coding/video_coding.gypi
index 515c6bc..8d31a6a 100644
--- a/webrtc/modules/video_coding/video_coding.gypi
+++ b/webrtc/modules/video_coding/video_coding.gypi
@@ -33,6 +33,7 @@
         'fec_tables_xor.h',
         'frame_buffer.h',
         'frame_object.h',
+        'rtp_frame_reference_finder.h',
         'generic_decoder.h',
         'generic_encoder.h',
         'histogram.h',
@@ -62,6 +63,7 @@
         'encoded_frame.cc',
         'frame_buffer.cc',
         'frame_object.cc',
+        'rtp_frame_reference_finder.cc',
         'generic_decoder.cc',
         'generic_encoder.cc',
         'inter_frame_delay.cc',