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

gm/aaclip.cpp

 /*
  * Copyright 2011 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 "SkPath.h"
@@ skipping 106 matching lines @@
     SkColor colors[] = { SK_ColorBLACK, SK_ColorBLACK, SK_ColorRED, SK_ColorRED };
     SkScalar pos[] = { 0, 0.3f, 0.3f, 1.0f };
     SkShader* s = SkGradientShader::CreateLinear(pts, colors, pos, 4, SkShader::kClamp_TileMode);
     SkPaint p;
     p.setShader(s)->unref();
     canvas->drawPaint(p);
 }
 
 static SkCanvas* MakeCanvas(const SkIRect& 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* canvas = new SkCanvas(bm);
     canvas->translate(-SkIntToScalar(bounds.fLeft), -SkIntToScalar(bounds.fTop));
     return canvas;
 }
 
 #ifdef SK_DEBUG
 static void GetBitmap(const SkCanvas* canvas, SkBitmap* bm) {
     *bm = canvas->getDevice()->accessBitmap(false);
@@ skipping 172 matching lines @@
         draw_rect_tests(canvas);
     }
 
     virtual uint32_t onGetFlags() const { return kSkipPipe_Flag; }
 
 private:
     typedef skiagm::GM INHERITED;
 };
 
 DEF_GM( return SkNEW(AAClipGM); )