Revert of Revert of Implement DisplayList GatherPixelRefs

cc/resources/display_item_list.cc

Index: cc/resources/display_item_list.cc
diff --git a/cc/resources/display_item_list.cc b/cc/resources/display_item_list.cc
index f8ffbfd184b719d447447b35d58a867ec42d3940..26ff003f0e535b1b603f0b33e612b0675d50fdd5 100644
--- a/cc/resources/display_item_list.cc
+++ b/cc/resources/display_item_list.cc
@@ -143,4 +143,14 @@
       "cc::DisplayItemList", this, AsValue());
 }
 
+void DisplayItemList::GatherPixelRefs(const gfx::Size& grid_cell_size) {
+  // This should be only called once, and only after CreateAndCacheSkPicture.
+  DCHECK(picture_);
+  DCHECK(!pixel_refs_);
+  pixel_refs_ = make_scoped_ptr(new PixelRefMap(grid_cell_size));
+  if (!picture_->willPlayBackBitmaps())
+    return;
+
+  pixel_refs_->GatherPixelRefsFromPicture(picture_.get());
+}
 }  // namespace cc