Add EncodedImageCallback::OnEncodedImage().

OnEncodedImage() is going to replace Encoded(), which is deprecated now.
The new OnEncodedImage() returns Result struct that contains frame_id,
which tells the encoder RTP timestamp for the frame.

BUG=chromium:621691
R=niklas.enbom@webrtc.org, sprang@webrtc.org, stefan@webrtc.org

Review URL: https://codereview.webrtc.org/2089773002 .

Cr-Commit-Position: refs/heads/master@{#13613}

webrtc/modules/video_coding/codecs/test/videoprocessor.h

@@ -230,7 +230,7 @@ class VideoProcessorImpl : public VideoProcessor {
    public:
     explicit VideoProcessorEncodeCompleteCallback(VideoProcessorImpl* vp)
         : video_processor_(vp) {}
-    int32_t Encoded(
+    Result OnEncodedImage(
         const webrtc::EncodedImage& encoded_image,
         const webrtc::CodecSpecificInfo* codec_specific_info,
         const webrtc::RTPFragmentationHeader* fragmentation) override;