gfx::ImageSkia: Added a note about using this class vs ImageFamily.

ui/gfx/image/image_skia.h

Index: ui/gfx/image/image_skia.h
diff --git a/ui/gfx/image/image_skia.h b/ui/gfx/image/image_skia.h
index 941ebc9faa8895fcd6f061f1bbee8425c177e5d5..7bf5e58fd57d74565795bef9991f09146bc03369 100644
--- a/ui/gfx/image/image_skia.h
+++ b/ui/gfx/image/image_skia.h
@@ -33,6 +33,11 @@ class TestOnThread;
 // Functions that mutate the image should operate on the gfx::ImageSkiaRep
 // returned from ImageSkia::GetRepresentation, not on ImageSkia.
 //
+// NOTE: This class should *not* be used to store multiple logical sizes of an
+// image (e.g., small, medium and large versions of an icon); use an ImageFamily
+// for that. An ImageSkia represents an image of a single logical size, with
+// potentially many different densities for high-DPI displays.
+//
 // ImageSkia is cheap to copy and intentionally supports copy semantics.
 class GFX_EXPORT ImageSkia {
  public: