Reduce public inheritance in VideoStreamEncoder.

Implementation of the EncodedImageCallback and
AdaptationObserverInterface belongs with the implementation, not the
public interface.

Bug: webrtc:8830
Change-Id: I901ae79ede252599c5fe22284cf649274c5b1422
Reviewed-on: https://webrtc-review.googlesource.com/51862
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22000}

video/video_stream_encoder.h

@@ -48,8 +48,9 @@ class VideoBitrateAllocationObserver;
 //  Call ConfigureEncoder with the codec settings.
 //  Call Stop() when done.
 class VideoStreamEncoder : public rtc::VideoSinkInterface<VideoFrame>,
-                           public EncodedImageCallback,
-                           public AdaptationObserverInterface {
+                           private EncodedImageCallback,
+                           // Protected only to provide access to tests.
+                           protected AdaptationObserverInterface {
  public:
   // Interface for receiving encoded video frames and notifications about
   // configuration changes.