cc: Add const versions of operator* to layer iterators.

cc/layers/picture_layer_impl.cc

Index: cc/layers/picture_layer_impl.cc
diff --git a/cc/layers/picture_layer_impl.cc b/cc/layers/picture_layer_impl.cc
index 4a7e0c0b0b66ae5bdf20743b65c0f51c69824c0d..ee9abc64bd15586e377110fd52c9df8cda19a1ee 100644
--- a/cc/layers/picture_layer_impl.cc
+++ b/cc/layers/picture_layer_impl.cc
@@ -1508,6 +1508,16 @@ Tile* PictureLayerImpl::LayerRasterTileIterator::operator*() {
   return *iterators_[index];
 }
 
+const Tile* PictureLayerImpl::LayerRasterTileIterator::operator*() const {
+  DCHECK(*this);
+
+  IteratorType index = stages_[current_stage_].iterator_type;
+  DCHECK(iterators_[index]);
+  DCHECK(iterators_[index].get_type() == stages_[current_stage_].tile_type);
+
+  return *iterators_[index];
+}
+
 PictureLayerImpl::LayerEvictionTileIterator::LayerEvictionTileIterator()
     : iterator_index_(0),
       iteration_stage_(TilePriority::EVENTUALLY),
@@ -1585,6 +1595,11 @@ Tile* PictureLayerImpl::LayerEvictionTileIterator::operator*() {
   return *iterators_[iterator_index_];
 }
 
+const Tile* PictureLayerImpl::LayerEvictionTileIterator::operator*() const {
+  DCHECK(*this);
+  return *iterators_[iterator_index_];
+}
+
 PictureLayerImpl::LayerEvictionTileIterator&
 PictureLayerImpl::LayerEvictionTileIterator::
 operator++() {