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

cc/test/fake_layer_tree_host.cc

Index: cc/test/fake_layer_tree_host.cc
diff --git a/cc/test/fake_layer_tree_host.cc b/cc/test/fake_layer_tree_host.cc
index 9e68fa337127e74b73d6108593dfb84d86ce0d91..ac270c9b938ba2da9e88f206737a96b229294380 100644
--- a/cc/test/fake_layer_tree_host.cc
+++ b/cc/test/fake_layer_tree_host.cc
@@ -89,7 +89,7 @@ void FakeLayerTreeHost::SetNeedsCommit() { needs_commit_ = true; }
 LayerImpl* FakeLayerTreeHost::CommitAndCreateLayerImplTree() {
   TreeSynchronizer::SynchronizeTrees(root_layer(), active_tree());
   active_tree()->SetPropertyTrees(property_trees());
-  TreeSynchronizer::PushLayerProperties(root_layer()->layer_tree_host(),
+  TreeSynchronizer::PushLayerProperties(root_layer()->GetLayerTree(),
                                         active_tree());
   animation_host()->PushPropertiesTo(host_impl_.animation_host());
 
@@ -111,7 +111,7 @@ LayerImpl* FakeLayerTreeHost::CommitAndCreateLayerImplTree() {
 LayerImpl* FakeLayerTreeHost::CommitAndCreatePendingTree() {
   TreeSynchronizer::SynchronizeTrees(root_layer(), pending_tree());
   pending_tree()->SetPropertyTrees(property_trees());
-  TreeSynchronizer::PushLayerProperties(root_layer()->layer_tree_host(),
+  TreeSynchronizer::PushLayerProperties(root_layer()->GetLayerTree(),
                                         pending_tree());
   animation_host()->PushPropertiesTo(host_impl_.animation_host());