Remove ViEEncoder::Pause / Start

This cl change so that VideoSendStream::Start adds the stream as a BitrateObserver and VideoSendStream::Stop removes the stream as observer.

That also means that start will trigger a VideoEncoder::SetRate call with the most recent bitrate estimate.
VideoSendStream::Stop will trigger a VideoEncoder::SetRate with bitrate  = 0.

BUG=webrtc:5687 b/28636240

Review-Url: https://codereview.webrtc.org/2070343002
Cr-Commit-Position: refs/heads/master@{#13192}
diff --git a/webrtc/modules/video_coding/video_coding_impl.h b/webrtc/modules/video_coding/video_coding_impl.h
index 2aabf05..6f48a23 100644
--- a/webrtc/modules/video_coding/video_coding_impl.h
+++ b/webrtc/modules/video_coding/video_coding_impl.h
@@ -96,7 +96,7 @@
   void Process() override;
 
  private:
-  void SetEncoderParameters(EncoderParameters params)
+  void SetEncoderParameters(EncoderParameters params, bool has_internal_source)
       EXCLUSIVE_LOCKS_REQUIRED(encoder_crit_);
 
   Clock* const clock_;