Don't cast picture ids (of type int64_t) to int.

Also cleaned up a bit in RtpFrameReferenceFinder.

Bug: chromium:762556
Change-Id: Ib08d2e7ce4b146b359ce9ba823f3aa15776c71bc
Reviewed-on: https://webrtc-review.googlesource.com/32301
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Commit-Queue: Philip Eliasson <philipel@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21282}
diff --git a/modules/video_coding/frame_buffer2.h b/modules/video_coding/frame_buffer2.h
index f535442..19fd94e 100644
--- a/modules/video_coding/frame_buffer2.h
+++ b/modules/video_coding/frame_buffer2.h
@@ -47,7 +47,7 @@
 
   // Insert a frame into the frame buffer. Returns the picture id
   // of the last continuous frame or -1 if there is no continuous frame.
-  int InsertFrame(std::unique_ptr<FrameObject> frame);
+  int64_t InsertFrame(std::unique_ptr<FrameObject> frame);
 
   // Get the next frame for decoding. Will return at latest after
   // |max_wait_time_ms|.