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

gm/dropshadowimagefilter.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 "SkColorFilter.h"
 
@@ skipping 36 matching lines @@
     canvas->restore();
 }
 
 static void draw_bitmap(SkCanvas* canvas, const SkRect& r, SkImageFilter* imf) {
     SkPaint paint;
 
     SkIRect bounds;
     r.roundOut(&bounds);
 
     SkBitmap bm;
-    bm.setConfig(SkBitmap::kARGB_8888_Config, bounds.width(), bounds.height());
-    bm.allocPixels();
+    bm.allocN32Pixels(bounds.width(), bounds.height());
     bm.eraseColor(SK_ColorTRANSPARENT);
     SkCanvas c(bm);
     draw_path(&c, r, NULL);
 
     paint.setImageFilter(imf);
     canvas->save();
     canvas->clipRect(r);
     canvas->drawBitmap(bm, 0, 0, &paint);
     canvas->restore();
 }
 
 static void draw_sprite(SkCanvas* canvas, const SkRect& r, SkImageFilter* imf) {
     SkPaint paint;
 
     SkIRect bounds;
     r.roundOut(&bounds);
 
     SkBitmap bm;
-    bm.setConfig(SkBitmap::kARGB_8888_Config, bounds.width(), bounds.height());
-    bm.allocPixels();
+    bm.allocN32Pixels(bounds.width(), bounds.height());
     bm.eraseColor(SK_ColorRED);
     SkCanvas c(bm);
 
     SkIRect cropRect = SkIRect::MakeXYWH(10, 10, 44, 44);
     paint.setColor(SK_ColorGREEN);
     c.drawRect(SkRect::Make(cropRect), paint);
 
     paint.setImageFilter(imf);
     SkPoint loc = { r.fLeft, r.fTop };
     canvas->getTotalMatrix().mapPoints(&loc, 1);
@@ skipping 76 matching lines @@
     }
 
 private:
     typedef GM INHERITED;
 };
 
 ///////////////////////////////////////////////////////////////////////////////
 
 static skiagm::GM* MyFactory(void*) { return new DropShadowImageFilterGM; }
 static skiagm::GMRegistry reg(MyFactory);