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.cc

@@ -157,4 +157,9 @@ bool VideoCodecUnitTest::WaitForDecodedFrame(std::unique_ptr<VideoFrame>* frame,
   }
 }
 
+size_t VideoCodecUnitTest::GetNumEncodedFrames() {
+  rtc::CritScope lock(&encoded_frame_section_);
+  return encoded_frames_.size();
+}
+
 }  // namespace webrtc