Avoid crashing when an ImageSkia has no representations and is converted to SkBitmap.

ui/gfx/image/image_skia_rep.h

Index: ui/gfx/image/image_skia_rep.h
diff --git a/ui/gfx/image/image_skia_rep.h b/ui/gfx/image/image_skia_rep.h
index 0376895ec4ed9c872fa3ed1fa73e13426802eb77..809ab98132419334c66892fd6094268505959359 100644
--- a/ui/gfx/image/image_skia_rep.h
+++ b/ui/gfx/image/image_skia_rep.h
@@ -46,6 +46,9 @@ class UI_EXPORT ImageSkiaRep {
   const SkBitmap& sk_bitmap() const { return bitmap_; }
 
  private:
+  friend class ImageSkia;
+  SkBitmap& mutable_sk_bitmap() { return bitmap_; }
+
   SkBitmap bitmap_;
   ui::ScaleFactor scale_factor_;
 };