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.cc b/modules/video_coding/frame_buffer2.cc
index 12bfd35..87fc498 100644
--- a/modules/video_coding/frame_buffer2.cc
+++ b/modules/video_coding/frame_buffer2.cc
@@ -285,7 +285,7 @@
     timing_->set_max_playout_delay(playout_delay.max_ms);
 }
 
-int FrameBuffer::InsertFrame(std::unique_ptr<FrameObject> frame) {
+int64_t FrameBuffer::InsertFrame(std::unique_ptr<FrameObject> frame) {
   TRACE_EVENT0("webrtc", "FrameBuffer::InsertFrame");
   RTC_DCHECK(frame);
   if (stats_callback_)
@@ -295,7 +295,7 @@
 
   rtc::CritScope lock(&crit_);
 
-  int last_continuous_picture_id =
+  int64_t last_continuous_picture_id =
       last_continuous_frame_it_ == frames_.end()
           ? -1
           : last_continuous_frame_it_->first.picture_id;