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

cc/layers/layer_impl.cc

Index: cc/layers/layer_impl.cc
diff --git a/cc/layers/layer_impl.cc b/cc/layers/layer_impl.cc
index 00df26a9fa93e849d44d5dbdbb9ddcdc9655a590..832fe51eda6dab83c3f20114901d3869e807ea5f 100644
--- a/cc/layers/layer_impl.cc
+++ b/cc/layers/layer_impl.cc
@@ -79,6 +79,8 @@ LayerImpl::LayerImpl(LayerTreeImpl* tree_impl, int id)
       current_draw_mode_(DRAW_MODE_NONE),
       mutable_properties_(MutableProperty::kNone),
       debug_info_(nullptr),
+      preferred_raster_scale_(1.0f),
+      has_preferred_raster_scale_(false),
       scrolls_drawn_descendant_(false),
       has_will_change_transform_hint_(false),
       needs_push_properties_(false) {
@@ -107,6 +109,16 @@ void LayerImpl::SetHasWillChangeTransformHint(bool has_will_change) {
   has_will_change_transform_hint_ = has_will_change;
 }
 
+void LayerImpl::SetPreferredRasterScale(float preferred_raster_scale) {
+  has_preferred_raster_scale_ = true;
+  preferred_raster_scale_ = preferred_raster_scale;
+}
+
+void LayerImpl::ClearPreferredRasterScale() {
+  has_preferred_raster_scale_ = false;
+  preferred_raster_scale_ = 1.0f;
+}
+
 AnimationHost* LayerImpl::GetAnimationHost() const {
   return layer_tree_impl_ ? layer_tree_impl_->animation_host() : nullptr;
 }