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

cc/trees/layer_tree_host_unittest_scroll.cc

 // Copyright 2012 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.
 
 #include "cc/trees/layer_tree_host.h"
 
 #include "base/location.h"
 #include "base/memory/weak_ptr.h"
 #include "base/single_thread_task_runner.h"
 #include "base/threading/thread_task_runner_handle.h"
@@ skipping 984 matching lines @@
   void BeginTest() override {
     layer_tree_host()->outer_viewport_scroll_layer()->SetScrollOffset(
         initial_scroll_);
     PostSetNeedsCommitToMainThread();
   }
 
   void WillCommit() override {
     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;
     }
   }
 
   void BeginCommitOnThread(LayerTreeHostImpl* impl) override {
     // Scroll after the 2nd commit has started.
     if (impl->active_tree()->source_frame_number() == 0) {
       LayerImpl* active_root = impl->active_tree()->root_layer_for_testing();
       LayerImpl* active_scroll_layer = impl->OuterViewportScrollLayer();
       ASSERT_TRUE(active_root);
@@ skipping 825 matching lines @@
   int num_impl_commits_;
   int num_aborted_commits_;
   int num_impl_scrolls_;
   int num_draws_;
 };
 
 MULTI_THREAD_TEST_F(LayerTreeHostScrollTestScrollAbortedCommitMFBA);
 
 }  // namespace
 }  // namespace cc