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

cc/layers/layer.cc

Index: cc/layers/layer.cc
diff --git a/cc/layers/layer.cc b/cc/layers/layer.cc
index e0e09b5d630c48532175ed6f7c0177344743c140..aafc73eb0830bfe8d8cb136b89962fb5e6180fa1 100644
--- a/cc/layers/layer.cc
+++ b/cc/layers/layer.cc
@@ -129,7 +129,7 @@ void Layer::SetLayerTreeHost(LayerTreeHost* host) {
   if (layer_tree_host_) {
     layer_tree_host_->property_trees()->RemoveIdFromIdToIndexMaps(id());
     layer_tree_host_->property_trees()->needs_rebuild = true;
-    layer_tree_host_->UnregisterLayer(this);
+    GetLayerTree()->UnregisterLayer(this);
     if (inputs_.element_id) {
       layer_tree_host_->animation_host()->UnregisterElement(
           inputs_.element_id, ElementListType::ACTIVE);
@@ -138,7 +138,7 @@ void Layer::SetLayerTreeHost(LayerTreeHost* host) {
   }
   if (host) {
     host->property_trees()->needs_rebuild = true;
-    host->RegisterLayer(this);
+    host->GetLayerTree()->RegisterLayer(this);
     if (inputs_.element_id) {
       host->AddToElementMap(this);
       host->animation_host()->RegisterElement(inputs_.element_id,
@@ -215,18 +215,18 @@ void Layer::SetNextCommitWaitsForActivation() {
 
 void Layer::SetNeedsPushProperties() {
   if (layer_tree_host_)

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

if (GetLayerTree()) here and throughout

-    layer_tree_host_->AddLayerShouldPushProperties(this);
+    GetLayerTree()->AddLayerShouldPushProperties(this);
 }
 
 void Layer::ResetNeedsPushPropertiesForTesting() {
-  layer_tree_host_->RemoveLayerShouldPushProperties(this);
+  GetLayerTree()->RemoveLayerShouldPushProperties(this);
 }
 
 bool Layer::IsPropertyChangeAllowed() const {
   if (!layer_tree_host_)
     return true;
 
-  return !layer_tree_host_->in_paint_layer_contents();
+  return !GetLayerTree()->in_paint_layer_contents();
 }
 
 sk_sp<SkPicture> Layer::GetPicture() const {
@@ -1195,7 +1195,7 @@ void Layer::PushPropertiesTo(LayerImpl* layer) {
   layer_property_changed_ = false;
   inputs_.update_rect = gfx::Rect();
 
-  layer_tree_host()->RemoveLayerShouldPushProperties(this);
+  GetLayerTree()->RemoveLayerShouldPushProperties(this);
 }
 
 void Layer::TakeCopyRequests(
@@ -1247,7 +1247,7 @@ void Layer::ClearLayerTreePropertiesForDeserializationAndAddToMap(
   (*layer_map)[inputs_.layer_id] = this;
 
   if (layer_tree_host_)
-    layer_tree_host_->UnregisterLayer(this);
+    GetLayerTree()->UnregisterLayer(this);
 
   layer_tree_host_ = nullptr;
   parent_ = nullptr;
@@ -1285,7 +1285,7 @@ void Layer::FromLayerNodeProto(const proto::LayerNode& proto,
   inputs_.layer_id = proto.id();
 
   layer_tree_host_ = layer_tree_host;
-  layer_tree_host_->RegisterLayer(this);
+  GetLayerTree()->RegisterLayer(this);
 
   for (int i = 0; i < proto.children_size(); ++i) {
     const proto::LayerNode& child_proto = proto.children(i);
@@ -1747,7 +1747,8 @@ void Layer::SetHasWillChangeTransformHint(bool has_will_change) {
 }
 
 AnimationHost* Layer::GetAnimationHost() const {
-  return layer_tree_host_ ? layer_tree_host_->animation_host() : nullptr;
+  LayerTree* layer_tree = GetLayerTree();
+  return layer_tree ? layer_tree->animation_host() : nullptr;
 }
 
 ElementListType Layer::GetElementTypeForAnimation() const {
@@ -1849,4 +1850,8 @@ gfx::Transform Layer::screen_space_transform() const {
       this, layer_tree_host_->property_trees()->transform_tree);
 }
 
+LayerTree* Layer::GetLayerTree() const {
+  return layer_tree_host_ ? layer_tree_host_->GetLayerTree() : nullptr;

[vmpstr, 2016/07/25 20:45:46]

If we're mimicking impl side of things, then layer_tree_impl is passed into the
ctor of layer_impl. Can we do a similar thing for the main side?

LayerTreeHost owns LayerTree which owns Layers, so it's a bit awkward for

Layer to talk to LayerTreeHost in order to get its LayerTree.

Most, if not all, interactions on the impl side happen from LayerImpl to
LayerTreeImpl, which may or may not forward the call to LayerTreeHostImpl. Is
this the plan in the future?

[xingliu, 2016/07/25 23:40:25]

@Khushal

Khushal probably can answer this question much better than me.

[Khushal, 2016/07/26 18:10:41]

Not sure. Blink uses the WebCompositorSupportImpl to build the WebLayers
wrapping the cc::Layers and that's shared across the RWCs which have the LTH. In
Clank UI as well, they just create Layers and don't have access to the LTH,
since its owned by the CompositorImpl in content/, so you just set the root
layer on the compositor which gives it to the host and it attaches itself to the
underlying tree. I figured the reason we have the SetLayerTreeHost method on
Layers is because you can construct one and then attach or detach it from the
LTH or change hosts? LayerImpls on the other hand remain bound to the
LayerTreeImpl right?
Yup, LayerTreeHost wouldn't exist, so SetLayerTreeHost would change to
SetLayerTree.

+}
+
 }  // namespace cc