Setup LayerTree class, refactor 2 functions from LayerTreeHost to it.

cc/layers/picture_layer.cc

Index: cc/layers/picture_layer.cc
diff --git a/cc/layers/picture_layer.cc b/cc/layers/picture_layer.cc
index 081e3158841943ab15a3d8bfaeff318afe457f5e..0a3d9761e9db1f4cbc0d73c16418d046485c00b8 100644
--- a/cc/layers/picture_layer.cc
+++ b/cc/layers/picture_layer.cc
@@ -82,7 +82,7 @@ void PictureLayer::SetLayerTreeHost(LayerTreeHost* host) {
 }
 
 void PictureLayer::SetNeedsDisplayRect(const gfx::Rect& layer_rect) {
-  DCHECK(!layer_tree_host() || !layer_tree_host()->in_paint_layer_contents());
+  DCHECK(!layer_tree_host() || !GetLayerTree()->in_paint_layer_contents());
   if (recording_source_)
     recording_source_->SetNeedsDisplayRect(layer_rect);
   Layer::SetNeedsDisplayRect(layer_rect);