Use vcm::VideoSender in ViEEncoder.

ViEEncoder doesn't need a full VideoCodingModule since it only uses the
sender side either way.

BUG=webrtc:3608,webrtc:5687
R=perkj@webrtc.org

Review URL: https://codereview.webrtc.org/1904983002 .

Cr-Commit-Position: refs/heads/master@{#12456}
diff --git a/webrtc/modules/video_coding/video_coding_impl.h b/webrtc/modules/video_coding/video_coding_impl.h
index e9d7abc..e3184a8 100644
--- a/webrtc/modules/video_coding/video_coding_impl.h
+++ b/webrtc/modules/video_coding/video_coding_impl.h
@@ -51,7 +51,7 @@
   int64_t _latestMs;
 };
 
-class VideoSender {
+class VideoSender : public Module {
  public:
   typedef VideoCodingModule::SenderNackMode SenderNackMode;
 
@@ -96,8 +96,8 @@
   void SuspendBelowMinBitrate();
   bool VideoSuspended() const;
 
-  int64_t TimeUntilNextProcess();
-  void Process();
+  int64_t TimeUntilNextProcess() override;
+  void Process() override;
 
  private:
   void SetEncoderParameters(EncoderParameters params)