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

cc/blink/web_layer_impl.h

Index: cc/blink/web_layer_impl.h
diff --git a/cc/blink/web_layer_impl.h b/cc/blink/web_layer_impl.h
index eaa2f05ae98da211c3d1cea1a09cdd581df94df0..28e15ff75f8ee7d926f6f57f213665444e9697bf 100644
--- a/cc/blink/web_layer_impl.h
+++ b/cc/blink/web_layer_impl.h
@@ -136,6 +136,8 @@ class CC_BLINK_EXPORT WebLayerImpl : public NON_EXPORTED_BASE(blink::WebLayer) {
   void setCompositorMutableProperties(uint32_t properties) override;
   uint32_t compositorMutableProperties() const override;
   void setHasWillChangeTransformHint(bool has_will_change) override;
+  void setPreferredRasterScale(float raster_scale) override;
+  void clearPreferredRasterScale() override;
 
   void setScrollParent(blink::WebLayer* parent) override;
   void setClipParent(blink::WebLayer* parent) override;