Add LayerTreeImpl end/begin/rend/rbegin

cc/trees/layer_tree_impl.cc

 // Copyright 2011 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_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 357 matching lines @@
 
   if (hud_layer())
     target_tree->set_hud_layer(static_cast<HeadsUpDisplayLayerImpl*>(
         LayerTreeHostCommon::FindLayerInSubtree(
             target_tree->root_layer(), hud_layer()->id())));
   else
     target_tree->set_hud_layer(NULL);
 
   target_tree->has_ever_been_drawn_ = false;
 }
+
+LayerListIterator LayerTreeImpl::begin() {
+  return LayerListIterator(root_layer_.get());
+}
+
+LayerListIterator LayerTreeImpl::end() {
+  return LayerListIterator(nullptr);
+}
+
+LayerListReverseIterator LayerTreeImpl::rbegin() {
+  return LayerListReverseIterator(root_layer_.get());
+}
+
+LayerListReverseIterator LayerTreeImpl::rend() {
+  return LayerListReverseIterator(nullptr);
+}
+
 void LayerTreeImpl::AddToElementMap(LayerImpl* layer) {
   if (!layer->element_id() || !layer->mutable_properties())
     return;
 
   TRACE_EVENT2(TRACE_DISABLED_BY_DEFAULT("compositor-worker"),
                "LayerTreeImpl::AddToElementMap", "element_id",
                layer->element_id(), "layer_id", layer->id());
 
   ElementLayers& layers = element_layers_map_[layer->element_id()];
   if ((!layers.main || layer->IsActive()) && !layer->scrollable()) {
@@ skipping 1648 matching lines @@
                                                    const gfx::BoxF& box,
                                                    gfx::BoxF* bounds) const {
   *bounds = gfx::BoxF();
   return layer_tree_host_impl_->animation_host()
              ? layer_tree_host_impl_->animation_host()
                    ->TransformAnimationBoundsForBox(layer->id(), box, bounds)
              : true;
 }
 
 }  // namespace cc