add legacy/helper allocN32Pixels, and convert gm to use it

gm/spritebitmap.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "gm.h"
 #include "SkCanvas.h"
 #include "SkBlurImageFilter.h"
 
 static void make_bm(SkBitmap* bm) {
-    bm->setConfig(SkBitmap::kARGB_8888_Config, 100, 100);
-    bm->allocPixels();
+    bm->allocN32Pixels(100, 100);
     bm->eraseColor(SK_ColorBLUE);
 
     SkCanvas canvas(*bm);
     SkPaint paint;
     paint.setAntiAlias(true);
     paint.setColor(SK_ColorRED);
     canvas.drawCircle(50, 50, 50, paint);
 }
 
 static void draw_2_bitmaps(SkCanvas* canvas, const SkBitmap& bm, bool doClip,
@@ skipping 65 matching lines @@
         draw_2_bitmaps(canvas, bm, true, dx, dy, filter);
     }
 
 private:
     typedef GM INHERITED;
 };
 
 //////////////////////////////////////////////////////////////////////////////
 
 DEF_GM( return new SpriteBitmapGM; )