Implement the logic to kick off image decoding jobs for TileManager

cc/picture_pile_impl.h

Index: cc/picture_pile_impl.h
diff --git a/cc/picture_pile_impl.h b/cc/picture_pile_impl.h
index 807bcbf4c47a8deea9a8eddbda35bf6db67e57e5..66dc5773dd10e155da902b748bf9c6b0c1f13635 100644
--- a/cc/picture_pile_impl.h
+++ b/cc/picture_pile_impl.h
@@ -28,6 +28,8 @@ public:
   // It is assumed that contentsScale has already been applied to this canvas.
   void Raster(SkCanvas* canvas, gfx::Rect rect, RenderingStats* stats);
 
+  std::vector<SkPixelRef*> GatherPixelRefs(const gfx::Rect& rect);
+
 private:
   friend class PicturePile;