Set correct encoder-specific settings for vpx in the new API.

Also, make VideoEncoderConfig::ContentType an enum class.

BUG=4569
R=mflodman@webrtc.org, pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/46069004

Cr-Commit-Position: refs/heads/master@{#9093}

webrtc/modules/video_coding/codecs/vp8/vp8_impl.cc

@@ -730,7 +730,7 @@ int VP8EncoderImpl::Encode(
   }
 
   // Only apply scaling to improve for single-layer streams. The scaling metrics
-  // use framedrops as a signal and is only applicable when we drop frames.
+  // use frame drops as a signal and is only applicable when we drop frames.
   const bool use_quality_scaler = encoders_.size() == 1 &&
                                   configurations_[0].rc_dropframe_thresh > 0 &&
                                   codec_.codecSpecific.VP8.automaticResizeOn;