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

cc/layers/layer.h

 // Copyright 2010 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.
 
 #ifndef CC_LAYERS_LAYER_H_
 #define CC_LAYERS_LAYER_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <set>
 #include <string>
 #include <unordered_map>
 #include <vector>
 
 #include "base/callback.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/observer_list.h"
 #include "cc/animation/element_id.h"
 #include "cc/animation/target_property.h"
 #include "cc/base/cc_export.h"
 #include "cc/base/region.h"
 #include "cc/debug/micro_benchmark.h"
 #include "cc/input/input_handler.h"
 #include "cc/layers/layer_collections.h"
 #include "cc/layers/layer_position_constraint.h"
 #include "cc/layers/paint_properties.h"
 #include "cc/output/filter_operations.h"
+#include "cc/trees/layer_tree.h"
 #include "cc/trees/mutator_host_client.h"
 #include "cc/trees/property_tree.h"
 #include "third_party/skia/include/core/SkColor.h"
 #include "third_party/skia/include/core/SkPicture.h"
 #include "third_party/skia/include/core/SkXfermode.h"
 #include "ui/gfx/geometry/point3_f.h"
 #include "ui/gfx/geometry/rect.h"
 #include "ui/gfx/geometry/rect_f.h"
 #include "ui/gfx/geometry/scroll_offset.h"
 #include "ui/gfx/transform.h"
@@ skipping 346 matching lines @@
   void ToLayerPropertiesProto(proto::LayerUpdate* layer_update);
 
   // Read all property values from the given LayerProperties object and update
   // the current layer. The values for |needs_push_properties_| and
   // |num_dependents_need_push_properties_| are always updated, but the rest
   // of |proto| is only read if |needs_push_properties_| is set.
   void FromLayerPropertiesProto(const proto::LayerProperties& proto);
 
   LayerTreeHost* layer_tree_host() { return layer_tree_host_; }
   const LayerTreeHost* layer_tree_host() const { return layer_tree_host_; }
+  LayerTree* GetLayerTree() const;

[vmpstr, 2016/07/27 18:05:30]

nit: Can you add a TODO to create an interface and this function will be a part
of it? I've talked to Khushal and I think that's the intent. 

[xingliu, 2016/07/28 16:10:24]

Done.

 
   virtual ScrollbarLayerInterface* ToScrollbarLayer();
 
   virtual sk_sp<SkPicture> GetPicture() const;
 
   // Constructs a LayerImpl of the correct runtime type for this Layer type.
   virtual std::unique_ptr<LayerImpl> CreateLayerImpl(LayerTreeImpl* tree_impl);
 
   bool NeedsDisplayForTesting() const { return !inputs_.update_rect.IsEmpty(); }
   void ResetNeedsDisplayForTesting() { inputs_.update_rect = gfx::Rect(); }
@@ skipping 313 matching lines @@
   // These all act like draw properties, so don't need push properties.
   gfx::Rect visible_layer_rect_;
   size_t num_unclipped_descendants_;
 
   DISALLOW_COPY_AND_ASSIGN(Layer);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_LAYER_H_