Break VP8 temporal_layers dependency on libvpx.

This is in preparation for
https://webrtc-review.googlesource.com/c/src/+/36340

With these changes we can avoid some strange #ifdefs in the code
that uses temporal layers.

Bug: webrtc:7925
Change-Id: I472210738ccc9f73812b8863951befeabec56f15
Reviewed-on: https://webrtc-review.googlesource.com/41280
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Anders Carlsson <andersc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21759}
diff --git a/modules/video_coding/codecs/vp8/screenshare_layers.h b/modules/video_coding/codecs/vp8/screenshare_layers.h
index 7143351..794b02b 100644
--- a/modules/video_coding/codecs/vp8/screenshare_layers.h
+++ b/modules/video_coding/codecs/vp8/screenshare_layers.h
@@ -45,7 +45,7 @@
 
   // Update the encoder configuration with target bitrates or other parameters.
   // Returns true iff the configuration was actually modified.
-  bool UpdateConfiguration(vpx_codec_enc_cfg_t* cfg) override;
+  bool UpdateConfiguration(Vp8EncoderConfig* cfg) override;
 
   void PopulateCodecSpecific(bool base_layer_sync,
                              const TemporalLayers::FrameConfig& tl_config,