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

cc/trees/occlusion_tracker.h

Index: cc/trees/occlusion_tracker.h
diff --git a/cc/trees/occlusion_tracker.h b/cc/trees/occlusion_tracker.h
index 8d07f434cf3b79ba409b0b27f73d4e927ef23c44..9b4b74e22fd228676381b4cf8e1b7a10e253eb7c 100644
--- a/cc/trees/occlusion_tracker.h
+++ b/cc/trees/occlusion_tracker.h
@@ -11,6 +11,7 @@
 #include "cc/base/cc_export.h"
 #include "cc/base/simple_enclosed_region.h"
 #include "cc/layers/layer_iterator.h"
+#include "cc/trees/occlusion.h"
 #include "ui/gfx/rect.h"
 
 namespace cc {
@@ -35,6 +36,11 @@ class CC_EXPORT OcclusionTracker {
   explicit OcclusionTracker(const gfx::Rect& screen_space_clip_rect);
   ~OcclusionTracker();
 
+  // Return an occlusion that retains the current state of the tracker
+  // and can be used outside of a layer walk to check occlusion.
+  Occlusion GetCurrentOcclusionForLayer(
+      const gfx::Transform& draw_transform) const;
+
   // Called at the beginning of each step in the LayerIterator's front-to-back
   // traversal.
   void EnterLayer(const LayerIteratorPosition<LayerType>& layer_iterator);