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 03c34bbced8bdcba39a1324140bda16d9d9dc267..340c0f2e1de16b95778c562955a2f46b167c25d6 100644
--- a/cc/trees/layer_tree_host_unittest_scroll.cc
+++ b/cc/trees/layer_tree_host_unittest_scroll.cc
@@ -1003,7 +1003,7 @@ class LayerTreeHostScrollTestImplOnlyScroll : public LayerTreeHostScrollTest {
     switch (layer_tree_host()->source_frame_number()) {
       case 0:
         EXPECT_TRUE(
-            scroll_layer->layer_tree_host()->LayerNeedsPushPropertiesForTesting(
+            scroll_layer->GetLayerTree()->LayerNeedsPushPropertiesForTesting(
                 scroll_layer));
         break;
       case 1:
@@ -1011,7 +1011,7 @@ class LayerTreeHostScrollTestImplOnlyScroll : public LayerTreeHostScrollTest {
         // still pick up scrolls that happen on the active layer during
         // commit.
         EXPECT_FALSE(
-            scroll_layer->layer_tree_host()->LayerNeedsPushPropertiesForTesting(
+            scroll_layer->GetLayerTree()->LayerNeedsPushPropertiesForTesting(
                 scroll_layer));
         break;
     }