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

cc/trees/layer_tree_host_unittest_scroll.cc

Index: cc/trees/layer_tree_host_unittest_scroll.cc
diff --git a/cc/trees/layer_tree_host_unittest_scroll.cc b/cc/trees/layer_tree_host_unittest_scroll.cc
index b23df26210fca2e8af54aad154fba93b508478aa..572e665bdfe98b6c82d07df0760abbd35ff3d3a7 100644
--- a/cc/trees/layer_tree_host_unittest_scroll.cc
+++ b/cc/trees/layer_tree_host_unittest_scroll.cc
@@ -1002,17 +1002,17 @@ class LayerTreeHostScrollTestImplOnlyScroll : public LayerTreeHostScrollTest {
     Layer* scroll_layer = layer_tree_host()->outer_viewport_scroll_layer();
     switch (layer_tree_host()->source_frame_number()) {
       case 0:
-        EXPECT_TRUE(
-            scroll_layer->layer_tree_host()->LayerNeedsPushPropertiesForTesting(
-                scroll_layer));
+        EXPECT_TRUE(scroll_layer->layer_tree_host()
+                        ->GetLayerTree()
+                        ->LayerNeedsPushPropertiesForTesting(scroll_layer));
         break;
       case 1:
         // Even if this layer doesn't need push properties, it should
         // still pick up scrolls that happen on the active layer during
         // commit.
-        EXPECT_FALSE(
-            scroll_layer->layer_tree_host()->LayerNeedsPushPropertiesForTesting(
-                scroll_layer));
+        EXPECT_FALSE(scroll_layer->layer_tree_host()
+                         ->GetLayerTree()
+                         ->LayerNeedsPushPropertiesForTesting(scroll_layer));
         break;
     }
   }