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

ui/gfx/image/image.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/gfx/image/image.h"
 
 #include <algorithm>
 #include <set>
 #include <utility>
 
@@ skipping 322 matching lines @@
  private:
   NSImage* image_;
 
   DISALLOW_COPY_AND_ASSIGN(ImageRepCocoa);
 };
 #endif  // defined(OS_MACOSX)
 
 // 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)
       : default_representation_type_(default_type)
 #if defined(OS_MACOSX) && !defined(OS_IOS)
         ,
         default_representation_color_space_(
             base::mac::GetGenericRGBColorSpace())
 #endif  // defined(OS_MACOSX) && !defined(OS_IOS)
   {
@@ skipping 408 matching lines @@
       storage_->representations().insert(std::make_pair(type, std::move(rep)));
 
   // insert should not fail (implies that there was already a representation of
   // that type in the map).
   CHECK(result.second) << "type was already in map.";
 
   return result.first->second.get();
 }
 
 }  // namespace gfx