Create default implementation to fix issue in libjingle

BUG=
R=mflodman@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/4509004

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@5181 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/video_engine/include/vie_image_process.h b/video_engine/include/vie_image_process.h
index 5006eda..aff2d61 100644
--- a/video_engine/include/vie_image_process.h
+++ b/video_engine/include/vie_image_process.h
@@ -99,12 +99,12 @@
 
   virtual void RegisterPostEncodeImageCallback(
       int video_channel,
-      EncodedImageCallback* post_encode_callback) = 0;
+      EncodedImageCallback* post_encode_callback) {}
   virtual void DeRegisterPostEncodeCallback(int video_channel) {}
 
   virtual void RegisterPreDecodeImageCallback(
       int video_channel,
-      EncodedImageCallback* pre_decode_callback) = 0;
+      EncodedImageCallback* pre_decode_callback) {}
   virtual void DeRegisterPreDecodeCallback(int video_channel) {}
 
   virtual void RegisterPreRenderCallback(