Replace VideoCapturerInput with VideoSinkInterface.

Adds new method VideoSendStream::SetSource(rtc::VideoSourceInterface* and VieEncoder::SetSource(rtc::VideoSourceInterface*)

This is the first step needed in order for the ViEEncoder to request downscaling using rtc::VideoSinkWants instead of separately reporting CPU overuse and internally doing downscaling due to QP values.

BUG=webrtc:5687
// Android CQ seems broken.
NOTRY=true

Review-Url: https://codereview.webrtc.org/2257413002
Cr-Commit-Position: refs/heads/master@{#14238}

webrtc/video/video_send_stream.h

@@ -71,7 +71,10 @@ class VideoSendStream : public webrtc::VideoSendStream {
   // webrtc::VideoSendStream implementation.
   void Start() override;
   void Stop() override;
-  VideoCaptureInput* Input() override;
+
+  void SetSource(
+      rtc::VideoSourceInterface<webrtc::VideoFrame>* source) override;
+
   void ReconfigureVideoEncoder(VideoEncoderConfig) override;
   Stats GetStats() override;