Revert of Revert of Implement DisplayList GatherPixelRefs

cc/resources/display_item_list.h

Index: cc/resources/display_item_list.h
diff --git a/cc/resources/display_item_list.h b/cc/resources/display_item_list.h
index 39694d72a1dba7ed8d5ecd444bc597a50941a5b4..f49055209d79fa762392547cb6d1cd3c547b96b1 100644
--- a/cc/resources/display_item_list.h
+++ b/cc/resources/display_item_list.h
@@ -11,6 +11,7 @@
 #include "cc/base/cc_export.h"
 #include "cc/base/scoped_ptr_vector.h"
 #include "cc/resources/display_item.h"
+#include "cc/resources/pixel_ref_map.h"
 #include "skia/ext/refptr.h"
 #include "third_party/skia/include/core/SkPicture.h"
 #include "ui/gfx/geometry/rect.h"
@@ -44,6 +45,8 @@
 
   void EmitTraceSnapshot() const;
 
+  void GatherPixelRefs(const gfx::Size& grid_cell_size);
+
  private:
   DisplayItemList();
   ~DisplayItemList();
@@ -54,7 +57,10 @@
   bool is_suitable_for_gpu_rasterization_;
   int approximate_op_count_;
 
+  scoped_ptr<PixelRefMap> pixel_refs_;
+
   friend class base::RefCountedThreadSafe<DisplayItemList>;
+  friend class PixelRefMap::Iterator;
   DISALLOW_COPY_AND_ASSIGN(DisplayItemList);
 };