Find root scroll layer at tree activation

cc/top_controls_manager_unittest.cc

Index: cc/top_controls_manager_unittest.cc
diff --git a/cc/top_controls_manager_unittest.cc b/cc/top_controls_manager_unittest.cc
index d081042f99404c05904bbdb2b988023f4de5de5d..4a8de9aa970306962fde4eacc25745f31e4260e5 100644
--- a/cc/top_controls_manager_unittest.cc
+++ b/cc/top_controls_manager_unittest.cc
@@ -27,7 +27,6 @@ class MockTopControlsManagerClient : public TopControlsManagerClient {
         update_draw_properties_needed_(false) {
     active_tree_ = LayerTreeImpl::create(&host_impl_);
     root_scroll_layer_ = LayerImpl::create(active_tree_.get(), 1);
-    active_tree_->set_root_scroll_layer(root_scroll_layer_.get());
   }
 
   virtual ~MockTopControlsManagerClient() {}
@@ -40,8 +39,8 @@ class MockTopControlsManagerClient : public TopControlsManagerClient {
     update_draw_properties_needed_ = true;
   }
 
-  virtual LayerTreeImpl* activeTree() OVERRIDE {
-    return active_tree_.get();
+  virtual LayerImpl* rootScrollLayer() const OVERRIDE {
+    return root_scroll_layer_.get();
   }
 
   TopControlsManager* manager() {
@@ -50,10 +49,6 @@ class MockTopControlsManagerClient : public TopControlsManagerClient {
     return manager_.get();
   }
 
-  LayerImpl* rootScrollLayer() {
-    return root_scroll_layer_.get();
-  }
-
  private:
   FakeImplProxy proxy_;
   FakeLayerTreeHostImpl host_impl_;