Framerate controller for VP9 screen sharing.

- Limit framerate by dropping frames before encoding.
- The max framerate at screen sharing is set to 5fps.

Bug: webrtc:9261
Change-Id: Icfbbecce33fdce2d746291708db0108e0ba10760
Reviewed-on: https://webrtc-review.googlesource.com/76921
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23316}

modules/video_coding/codecs/vp9/vp9_impl.h

@@ -17,6 +17,7 @@
 
 #include "modules/video_coding/codecs/vp9/include/vp9.h"
 #include "modules/video_coding/codecs/vp9/vp9_frame_buffer_pool.h"
+#include "rtc_base/rate_statistics.h"
 
 #include "vpx/vp8cx.h"
 #include "vpx/vpx_decoder.h"
@@ -103,6 +104,11 @@ class VP9EncoderImpl : public VP9Encoder {
   uint8_t num_spatial_layers_;
   InterLayerPredMode inter_layer_pred_;
 
+  // Framerate controller.
+  rtc::Optional<float> target_framerate_fps_;
+  RateStatistics output_framerate_;
+  uint32_t last_encoded_frame_rtp_timestamp_;
+
   // Used for flexible mode.
   bool is_flexible_mode_;
 };