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/test/video_codec_unittest.h

@@ -99,6 +99,8 @@ class VideoCodecUnitTest : public ::testing::Test {
   bool WaitForDecodedFrame(std::unique_ptr<VideoFrame>* frame,
                            rtc::Optional<uint8_t>* qp);
 
+  size_t GetNumEncodedFrames();
+
   VideoCodec codec_settings_;
 
   std::unique_ptr<VideoEncoder> encoder_;