Revert of Revert of Implement DisplayList GatherPixelRefs

cc/resources/display_list_raster_source.cc

Index: cc/resources/display_list_raster_source.cc
diff --git a/cc/resources/display_list_raster_source.cc b/cc/resources/display_list_raster_source.cc
index e35900a065f7637f8f3aa3ceb23d3b3518687fb3..9a0bc9111506c0c92b61e447c8ac1a572b41f519 100644
--- a/cc/resources/display_list_raster_source.cc
+++ b/cc/resources/display_list_raster_source.cc
@@ -159,7 +159,16 @@
     const gfx::Rect& content_rect,
     float contents_scale,
     std::vector<SkPixelRef*>* pixel_refs) const {
-  // TODO(ajuma): Implement this.
+  DCHECK_EQ(0u, pixel_refs->size());
+
+  gfx::Rect layer_rect =
+      gfx::ScaleToEnclosingRect(content_rect, 1.0f / contents_scale);
+
+  PixelRefMap::Iterator iterator(layer_rect, display_list_.get());
+  while (iterator) {
+    pixel_refs->push_back(*iterator);
+    ++iterator;
+  }
 }
 
 bool DisplayListRasterSource::CoversRect(const gfx::Rect& content_rect,