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

ui/gfx/skbitmap_operations.h

Index: ui/gfx/skbitmap_operations.h
diff --git a/ui/gfx/skbitmap_operations.h b/ui/gfx/skbitmap_operations.h
index fd25d5efc977d847c2e112f06b99b3239456fc26..0850188f0b857a5596ae85c4994968e5ab94fbca 100644
--- a/ui/gfx/skbitmap_operations.h
+++ b/ui/gfx/skbitmap_operations.h
@@ -32,12 +32,6 @@ class GFX_EXPORT SkBitmapOperations {
   // (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.