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

gm/verylargebitmap.cpp

 /*
  * Copyright 2012 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 "SkGradientShader.h"
 #include "SkPath.h"
 
 static void make_bm(SkBitmap* bm, int width, int height, SkColor colors[2]) {
-    bm->setConfig(SkBitmap::kARGB_8888_Config, width, height);
-    bm->allocPixels();
+    bm->allocN32Pixels(width, height);
     SkCanvas canvas(*bm);
     SkPoint center = {SkIntToScalar(width)/2, SkIntToScalar(height)/2};
     SkScalar radius = 40;
     SkShader* shader = SkGradientShader::CreateRadial(center, radius, colors, NULL, 2,
                                                       SkShader::kMirror_TileMode);
     SkPaint paint;
     paint.setShader(shader)->unref();
     paint.setXfermodeMode(SkXfermode::kSrc_Mode);
     canvas.drawPaint(paint);
     bm->setImmutable();
@@ skipping 92 matching lines @@
     typedef skiagm::GM INHERITED;
 };
 
 //////////////////////////////////////////////////////////////////////////////
 
 // This GM allocates more memory than Android devices are capable of fulfilling.
 #ifndef SK_BUILD_FOR_ANDROID
 static skiagm::GM* MyFactory(void*) { return new VeryLargeBitmapGM; }
 static skiagm::GMRegistry reg(MyFactory);
 #endif