Make RootScroller set the outer viewport scroll layer in the compositor

cc/trees/root_scroller_controller.cc

+// Copyright 2016 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/root_scroller_controller.h"
+
+#include "base/memory/ptr_util.h"
+
+namespace cc {
+
+// static
+std::unique_ptr<RootScrollerController> RootScrollerController::Create() {
+  return base::WrapUnique(new RootScrollerController);
+}
+
+RootScrollerController::RootScrollerController()
+    : root_scroller_layer_(nullptr) {}
+
+void RootScrollerController::SetRootScrollerLayer(LayerImpl* layer) {
+  root_scroller_layer_ = layer;
+}
+
+LayerImpl* RootScrollerController::GetRootScrollerLayer() {
+  return root_scroller_layer_;
+}
+
+}  // namespace cc