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

cc/trees/occlusion_tracker.cc

 // Copyright 2012 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/occlusion_tracker.h"
 
 #include <algorithm>
 
 #include "cc/base/math_util.h"
 #include "cc/base/region.h"
@@ skipping 10 matching lines @@
 OcclusionTracker<LayerType>::OcclusionTracker(
     const gfx::Rect& screen_space_clip_rect)
     : screen_space_clip_rect_(screen_space_clip_rect),
       occluding_screen_space_rects_(NULL),
       non_occluding_screen_space_rects_(NULL) {}
 
 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;
 
   if (layer_iterator.represents_itself)
     EnterRenderTarget(render_target);
   else if (layer_iterator.represents_target_render_surface)
     FinishedRenderTarget(render_target);
 }
 
@@ skipping 579 matching lines @@
   for (size_t i = 0; i < occluded.GetRegionComplexity(); ++i)
     visible_region.Subtract(occluded.GetRect(i));
   return visible_region;
 }
 
 // Instantiate (and export) templates here for the linker.
 template class OcclusionTracker<Layer>;
 template class OcclusionTracker<LayerImpl>;
 
 }  // namespace cc