ui/gfx: remove redundant erase operations and remove unused function

ui/gfx/skbitmap_operations.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_SKBITMAP_OPERATIONS_H_
 #define UI_GFX_SKBITMAP_OPERATIONS_H_
 
 #include "base/gtest_prod_util.h"
 #include "ui/gfx/color_utils.h"
 #include "ui/gfx/gfx_export.h"
@@ skipping 14 matching lines @@
     ROTATION_90_CW,
     ROTATION_180_CW,
     ROTATION_270_CW,
   };
 
   // Create a bitmap that is an inverted image of the passed in image.
   // Each color becomes its inverse in the color wheel. So (255, 15, 0) becomes
   // (0, 240, 255). The alpha value is not inverted.
   static SkBitmap CreateInvertedBitmap(const SkBitmap& image);
 
-  // Create a bitmap that is a superimposition of the second bitmap on top of
-  // the first. The provided bitmaps must use have the kARGB_8888_Config config
-  // and be of equal dimensions.
-  static SkBitmap CreateSuperimposedBitmap(const SkBitmap& first,
-                                           const SkBitmap& second);
-
   // Create a bitmap that is a blend of two others. The alpha argument
   // specifies the opacity of the second bitmap. The provided bitmaps must
   // use have the kARGB_8888_Config config and be of equal dimensions.
   static SkBitmap CreateBlendedBitmap(const SkBitmap& first,
                                       const SkBitmap& second,
                                       double alpha);
 
   // Create a bitmap that is the original bitmap masked out by the mask defined
   // in the alpha bitmap. The images must use the kARGB_8888_Config config and
   // be of equal dimensions.
@@ skipping 70 matching lines @@
   static SkBitmap Rotate(const SkBitmap& source, RotationAmount rotation);
 
  private:
   SkBitmapOperations();  // Class for scoping only.
 
   FRIEND_TEST_ALL_PREFIXES(SkBitmapOperationsTest, DownsampleByTwo);
   FRIEND_TEST_ALL_PREFIXES(SkBitmapOperationsTest, DownsampleByTwoSmall);
 };
 
 #endif  // UI_GFX_SKBITMAP_OPERATIONS_H_