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

This is the source of some confusion; ImageSkia has been abused in the
past for storing different logical image sizes, where an ImageFamily is
the appropriate class. This comment mirrors the comment in ImageFamily
explaining when Image/ImageSkia is appropriate vs ImageFamily.

Review URL: https://codereview.chromium.org/1438603003

Cr-Commit-Position: refs/heads/master@{#361067}

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: