Rename adaptation api methods, extended vie_encoder unit test.

Use AdaptDown/AdaptUp instead of ScaleDown/ScaleUp, since we may want to
adapt using other means than resolution.

Also, extend vie_encoder with unit test that actually uses frames scaled
to resolution as determined by VideoAdapter, since that seems to be the
default implementation.

BUG=webrtc:4172

Review-Url: https://codereview.webrtc.org/2652893015
Cr-Commit-Position: refs/heads/master@{#16402}
diff --git a/webrtc/test/frame_generator.h b/webrtc/test/frame_generator.h
index cea3dda..e2519c1 100644
--- a/webrtc/test/frame_generator.h
+++ b/webrtc/test/frame_generator.h
@@ -29,12 +29,13 @@
 class FrameForwarder : public rtc::VideoSourceInterface<VideoFrame> {
  public:
   FrameForwarder();
+  virtual ~FrameForwarder();
   // Forwards |video_frame| to the registered |sink_|.
-  void IncomingCapturedFrame(const VideoFrame& video_frame);
+  virtual void IncomingCapturedFrame(const VideoFrame& video_frame);
   rtc::VideoSinkWants sink_wants() const;
   bool has_sinks() const;
 
- private:
+ protected:
   void AddOrUpdateSink(rtc::VideoSinkInterface<VideoFrame>* sink,
                        const rtc::VideoSinkWants& wants) override;
   void RemoveSink(rtc::VideoSinkInterface<VideoFrame>* sink) override;