Reland of: cc: Add occlusion checker as a fixed view of occlusion tracker.

cc/trees/occlusion_tracker.cc

Index: cc/trees/occlusion_tracker.cc
diff --git a/cc/trees/occlusion_tracker.cc b/cc/trees/occlusion_tracker.cc
index f233e074a9ba8b0a1441658ad2751ec3dbd907af..8fdd6e256fad0b0d5543883c544b8878446cf408 100644
--- a/cc/trees/occlusion_tracker.cc
+++ b/cc/trees/occlusion_tracker.cc
@@ -28,6 +28,16 @@ template <typename LayerType>
 OcclusionTracker<LayerType>::~OcclusionTracker() {}
 
 template <typename LayerType>
+Occlusion OcclusionTracker<LayerType>::GetCurrentOcclusionForLayer(
+    const gfx::Transform& draw_transform) const {
+  DCHECK(!stack_.empty());
+  const StackObject& back = stack_.back();
+  return Occlusion(draw_transform,
+                   back.occlusion_from_outside_target,
+                   back.occlusion_from_inside_target);
+}
+
+template <typename LayerType>
 void OcclusionTracker<LayerType>::EnterLayer(
     const LayerIteratorPosition<LayerType>& layer_iterator) {
   LayerType* render_target = layer_iterator.target_render_surface_layer;