Document how effective zoom relates to StyleImage sizing

third_party/WebKit/Source/core/paint/NinePieceImagePainter.cpp

Index: third_party/WebKit/Source/core/paint/NinePieceImagePainter.cpp
diff --git a/third_party/WebKit/Source/core/paint/NinePieceImagePainter.cpp b/third_party/WebKit/Source/core/paint/NinePieceImagePainter.cpp
index 51d82366c8513042c31c0e23ca7ca5ee4d94cca8..3eae2c371d3ab249e6935e94f82e9f2271dd48e0 100644
--- a/third_party/WebKit/Source/core/paint/NinePieceImagePainter.cpp
+++ b/third_party/WebKit/Source/core/paint/NinePieceImagePainter.cpp
@@ -49,6 +49,8 @@ bool NinePieceImagePainter::paint(GraphicsContext& graphicsContext, const Layout
     rectWithOutsets.expand(style.imageOutsets(ninePieceImage));
     LayoutRect borderImageRect = rectWithOutsets;
 
+    // NinePieceImage returns the image slices without effective zoom applied and thus we compute
+    // the nine piece grid on top of the image in unzoomed coordinates.
     LayoutSize defaultObjectSize = borderImageRect.size();
     defaultObjectSize.scale(1 / style.effectiveZoom());
     IntSize imageSize = roundedIntSize(styleImage->imageSize(m_layoutObject, 1, defaultObjectSize));