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

cc/blink/web_layer_impl.cc

Index: cc/blink/web_layer_impl.cc
diff --git a/cc/blink/web_layer_impl.cc b/cc/blink/web_layer_impl.cc
index ba40a266e7526a44cc380ca84568bb8aa6ac39cb..4d4f9936b5218e6e8a129640f6e32862074bcca5 100644
--- a/cc/blink/web_layer_impl.cc
+++ b/cc/blink/web_layer_impl.cc
@@ -474,4 +474,12 @@ void WebLayerImpl::setHasWillChangeTransformHint(bool has_will_change) {
   layer_->SetHasWillChangeTransformHint(has_will_change);
 }
 
+void WebLayerImpl::setPreferredRasterScale(float raster_scale) {
+  layer_->SetPreferredRasterScale(raster_scale);
+}
+
+void WebLayerImpl::clearPreferredRasterScale() {
+  layer_->ClearPreferredRasterScale();
+}
+
 }  // namespace cc_blink