Add a comment about gfx::internal::ImageStorage and RefCountedThreadSafe.

ui/gfx/image/image.cc

Index: ui/gfx/image/image.cc
diff --git a/ui/gfx/image/image.cc b/ui/gfx/image/image.cc
index 12254c17def66a5b46fb45db8257732cf14af173..b2984fb0f3d844254aa8ff409fa8cd3780f42d9c 100644
--- a/ui/gfx/image/image.cc
+++ b/ui/gfx/image/image.cc
@@ -340,6 +340,10 @@ class ImageRepCocoa : public ImageRep {
 // The Storage class acts similarly to the pixels in a SkBitmap: the Image
 // class holds a refptr instance of Storage, which in turn holds all the
 // ImageReps. This way, the Image can be cheaply copied.
+//
+// This class is deliberately not RefCountedThreadSafe. Making it so does not
+// solve threading issues, as gfx::Image and its internal classes are
+// themselves not threadsafe.
 class ImageStorage : public base::RefCounted<ImageStorage> {
  public:
   ImageStorage(Image::RepresentationType default_type)