Plumb preferred raster scale for background images from Blink to cc layers.

cc/layers/layer_impl.h

Index: cc/layers/layer_impl.h
diff --git a/cc/layers/layer_impl.h b/cc/layers/layer_impl.h
index 478fd00948f5f49f960b9568e1ed434ad545702a..01de99cfc84fe670b9c369bedc1cb6f1455b7012 100644
--- a/cc/layers/layer_impl.h
+++ b/cc/layers/layer_impl.h
@@ -447,6 +447,11 @@ class CC_EXPORT LayerImpl {
     return has_will_change_transform_hint_;
   }
 
+  void SetPreferredRasterScale(float raster_scale);
+  bool has_preferred_raster_scale() { return has_preferred_raster_scale_; }
+  float preferred_raster_scale() const { return preferred_raster_scale_; }
+  void ClearPreferredRasterScale();
+
   AnimationHost* GetAnimationHost() const;
 
   ElementListType GetElementTypeForAnimation() const;
@@ -563,7 +568,9 @@ class CC_EXPORT LayerImpl {
       owned_debug_info_;
   base::trace_event::ConvertableToTraceFormat* debug_info_;
   std::unique_ptr<RenderSurfaceImpl> render_surface_;
+  float preferred_raster_scale_;
 
+  bool has_preferred_raster_scale_ : 1;
   bool scrolls_drawn_descendant_ : 1;
   bool has_will_change_transform_hint_ : 1;
   bool needs_push_properties_ : 1;