Change pre_encode_callback to get a const frame.

webrtc/video/vie_encoder.h

Index: webrtc/video/vie_encoder.h
diff --git a/webrtc/video/vie_encoder.h b/webrtc/video/vie_encoder.h
index ce1e508eba92760c65b46c8783aaea9561951661..799d4d5f59b03a3ff65a4189abb2aea1e5d2a5e5 100644
--- a/webrtc/video/vie_encoder.h
+++ b/webrtc/video/vie_encoder.h
@@ -18,7 +18,7 @@
 #include "webrtc/base/scoped_ref_ptr.h"
 #include "webrtc/base/thread_annotations.h"
 #include "webrtc/common_types.h"
-#include "webrtc/frame_callback.h"
+#include "webrtc/media/base/videosinkinterface.h"
 #include "webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h"
 #include "webrtc/modules/video_coding/include/video_coding_defines.h"
 #include "webrtc/modules/video_processing/include/video_processing.h"
@@ -49,7 +49,8 @@ class ViEEncoder : public VideoEncoderRateObserver,
              const std::vector<uint32_t>& ssrcs,
              ProcessThread* module_process_thread,
              SendStatisticsProxy* stats_proxy,
-             I420FrameCallback* pre_encode_callback,
+             // TODO(nisse): Used only for tests, delete?
+             rtc::VideoSinkInterface<VideoFrame>* pre_encode_callback,
              OveruseFrameDetector* overuse_detector,
              PacedSender* pacer,
              PayloadRouter* payload_router);
@@ -130,7 +131,7 @@ class ViEEncoder : public VideoEncoderRateObserver,
   rtc::CriticalSection data_cs_;
 
   SendStatisticsProxy* const stats_proxy_;
-  I420FrameCallback* const pre_encode_callback_;
+  rtc::VideoSinkInterface<VideoFrame>* const pre_encode_callback_;
   OveruseFrameDetector* const overuse_detector_;
   PacedSender* const pacer_;
   PayloadRouter* const send_payload_router_;