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

ui/gfx/image/image_skia_rep.h

 // 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.
 
 #ifndef UI_GFX_IMAGE_IMAGE_SKIA_REP_H_
 #define UI_GFX_IMAGE_IMAGE_SKIA_REP_H_
 
 #include "third_party/skia/include/core/SkBitmap.h"
 #include "ui/base/layout.h"
 #include "ui/base/ui_export.h"
@@ skipping 28 matching lines @@
   int pixel_height() const { return bitmap_.height(); }
 
   // Retrieves the scale that the bitmap will be painted at.
   float GetScale() const;
   ui::ScaleFactor scale_factor() const { return scale_factor_; }
 
   // Returns backing bitmap.
   const SkBitmap& sk_bitmap() const { return bitmap_; }
 
  private:
+  friend class ImageSkia;
+  SkBitmap& mutable_sk_bitmap() { return bitmap_; }
+
   SkBitmap bitmap_;
   ui::ScaleFactor scale_factor_;
 };
 
 }  // namespace gfx
 
 #endif  // UI_GFX_IMAGE_IMAGE_SKIA_REP_H_