Make it possible to custom-paint without an Element.

sky/engine/core/rendering/RenderBox.cpp

Index: sky/engine/core/rendering/RenderBox.cpp
diff --git a/sky/engine/core/rendering/RenderBox.cpp b/sky/engine/core/rendering/RenderBox.cpp
index 59acacea1dcec48cc0d953072615bd69f7344d5b..c7d70da18919727a3a876f49f452242438096332 100644
--- a/sky/engine/core/rendering/RenderBox.cpp
+++ b/sky/engine/core/rendering/RenderBox.cpp
@@ -994,19 +994,24 @@ BackgroundBleedAvoidance RenderBox::determineBackgroundBleedAvoidance(GraphicsCo
     return BackgroundBleedClipBackground;
 }
 
-void RenderBox::paintBoxDecorationBackground(PaintInfo& paintInfo, const LayoutPoint& paintOffset)
+void RenderBox::paintCustomPainting(PaintInfo& paintInfo, const LayoutPoint& paintOffset)
 {
     LayoutRect paintRect = borderBoxRect();
     paintRect.moveBy(paintOffset);
-    paintBoxDecorationBackgroundWithRect(paintInfo, paintOffset, paintRect);
     // TODO(abarth): Currently we only draw m_customPainting if we happen to
-    // have a box decoration or a background. Instead, we should probably have
-    // another function like paintBoxDecorationBackground that subclasses can
-    // call to draw m_customPainting.
+    // have a box decoration or a background.
     if (m_customPainting)
         paintInfo.context->drawDisplayList(m_customPainting.get(), paintRect.location());
 }
 
+void RenderBox::paintBoxDecorationBackground(PaintInfo& paintInfo, const LayoutPoint& paintOffset)
+{
+    LayoutRect paintRect = borderBoxRect();
+    paintRect.moveBy(paintOffset);
+    paintBoxDecorationBackgroundWithRect(paintInfo, paintOffset, paintRect);
+    paintCustomPainting(paintInfo, paintOffset);
+}
+
 void RenderBox::paintBoxDecorationBackgroundWithRect(PaintInfo& paintInfo, const LayoutPoint& paintOffset, const LayoutRect& paintRect)
 {
     RenderStyle* style = this->style();