cc: Remove LayerTreeImpl::root_layer usage outside tests

cc/trees/layer_tree_host_unittest_animation.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 <stdint.h>
 
 #include "cc/animation/animation_curve.h"
 #include "cc/animation/animation_host.h"
@@ skipping 1321 matching lines @@
   void SwapBuffersOnThread(LayerTreeHostImpl* host_impl, bool result) override {
     // After both animations have started, verify that they have valid
     // start times.
     if (host_impl->active_tree()->source_frame_number() < 2)
       return;
     AnimationHost::ElementToAnimationsMap element_animations_copy =
         host_impl->animation_host()->active_element_animations_for_testing();
     EXPECT_EQ(2u, element_animations_copy.size());
     for (auto& it : element_animations_copy) {
       int id = it.first;
-      if (id == host_impl->RootLayer()->id()) {
+      if (id == host_impl->active_tree()->root_layer()->id()) {
         Animation* anim = it.second->GetAnimation(TargetProperty::TRANSFORM);
         EXPECT_GT((anim->start_time() - base::TimeTicks()).InSecondsF(), 0);
       } else if (id == layer_->id()) {
         Animation* anim = it.second->GetAnimation(TargetProperty::OPACITY);
         EXPECT_GT((anim->start_time() - base::TimeTicks()).InSecondsF(), 0);
       }
       EndTest();
     }
   }
 
@@ skipping 418 matching lines @@
  private:
   scoped_refptr<Layer> layer_;
   FakeContentLayerClient client_;
 };
 
 MULTI_THREAD_TEST_F(
     LayerTreeHostAnimationTestRebuildPropertyTreesOnAnimationSetNeedsCommit);
 
 }  // namespace
 }  // namespace cc