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

cc/layers/picture_layer.cc

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/layers/picture_layer.h"
 
 #include "base/auto_reset.h"
 #include "base/trace_event/trace_event.h"
 #include "cc/layers/content_layer_client.h"
 #include "cc/layers/picture_layer_impl.h"
@@ skipping 64 matching lines @@
   recording_source_->SetSlowdownRasterScaleFactor(
       host->debug_state().slow_down_raster_scale_factor);
   // If we need to enable image decode tasks, then we have to generate the
   // discardable images metadata.
   const LayerTreeSettings& settings = layer_tree_host()->settings();
   recording_source_->SetGenerateDiscardableImagesMetadata(
       settings.image_decode_tasks_enabled);
 }
 
 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);
 }
 
 bool PictureLayer::Update() {
   update_source_frame_number_ = layer_tree_host()->source_frame_number();
   bool updated = Layer::Update();
 
   gfx::Size layer_size = paint_properties().bounds;
@@ skipping 156 matching lines @@
   if (update_source_frame_number_ != source_frame_number &&
       recording_source_bounds != layer_bounds) {
     // Update may not get called for the layer (if it's not in the viewport
     // for example), even though it has resized making the recording source no
     // longer valid. In this case just destroy the recording source.
     recording_source_->SetEmptyBounds();
   }
 }
 
 }  // namespace cc