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

gm/colormatrix.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 "SkColorMatrixFilter.h"
 #include "SkGradientShader.h"
@@ skipping 45 matching lines @@
     virtual SkString onShortName() {
         return SkString("colormatrix");
     }
 
     virtual SkISize onISize() {
         return make_isize(WIDTH, HEIGHT);
     }
 
     SkBitmap createSolidBitmap(int width, int height) {
         SkBitmap bm;
-        bm.setConfig(SkBitmap::kARGB_8888_Config, width, height);
-        bm.allocPixels();
+        bm.allocN32Pixels(width, height);
         SkCanvas canvas(bm);
         canvas.clear(0x0);
         for (int y = 0; y < height; ++y) {
             for (int x = 0; x < width; ++x) {
                 SkPaint paint;
                 paint.setColor(SkColorSetARGB(255, x * 255 / width, y * 255 / height, 0));
                 canvas.drawRect(SkRect::MakeXYWH(SkIntToScalar(x),
                     SkIntToScalar(y), SK_Scalar1, SK_Scalar1), paint);
             }
         }
         return bm;
     }
 
     // creates a bitmap with shades of transparent gray.
     SkBitmap createTransparentBitmap(int width, int height) {
         SkBitmap bm;
-        bm.setConfig(SkBitmap::kARGB_8888_Config, width, height);
-        bm.allocPixels();
+        bm.allocN32Pixels(width, height);
         SkCanvas canvas(bm);
         canvas.clear(0x0);
 
         SkPoint pts[] = {{0, 0}, {SkIntToScalar(width), SkIntToScalar(height)}};
         SkColor colors[] = {0x00000000, 0xFFFFFFFF};
         SkPaint paint;
         paint.setShader(SkGradientShader::CreateLinear(pts, colors, NULL, 2,
                                                        SkShader::kClamp_TileMode))->unref();
         canvas.drawRect(SkRect::MakeWH(SkIntToScalar(width), SkIntToScalar(height)), paint);
         return bm;
@@ skipping 72 matching lines @@
     SkBitmap fTransparentBitmap;
     typedef GM INHERITED;
 };
 
 //////////////////////////////////////////////////////////////////////////////
 
 static GM* MyFactory(void*) { return new ColorMatrixGM; }
 static GMRegistry reg(MyFactory);
 
 }