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

gm/xfermodeimagefilter.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 "SkArithmeticMode.h"
 #include "SkOffsetImageFilter.h"
@@ skipping 11 matching lines @@
     XfermodeImageFilterGM(){
         this->setBGColor(0xFF000000);
     }
 
 protected:
     virtual SkString onShortName() SK_OVERRIDE {
         return SkString("xfermodeimagefilter");
     }
 
     void make_bitmap() {
-        fBitmap.setConfig(SkBitmap::kARGB_8888_Config, 80, 80);
-        fBitmap.allocPixels();
+        fBitmap.allocN32Pixels(80, 80);
         SkBitmapDevice device(fBitmap);
         SkCanvas canvas(&device);
         canvas.clear(0x00000000);
         SkPaint paint;
         paint.setAntiAlias(true);
         paint.setColor(0xD000D000);
         paint.setTextSize(SkIntToScalar(96));
         const char* str = "e";
         canvas.drawText(str, strlen(str), SkIntToScalar(15), SkIntToScalar(65), paint);
     }
 
     void make_checkerboard() {
-        fCheckerboard.setConfig(SkBitmap::kARGB_8888_Config, 80, 80);
-        fCheckerboard.allocPixels();
+        fCheckerboard.allocN32Pixels(80, 80);
         SkBitmapDevice device(fCheckerboard);
         SkCanvas canvas(&device);
         canvas.clear(0x00000000);
         SkPaint darkPaint;
         darkPaint.setColor(0xFF404040);
         SkPaint lightPaint;
         lightPaint.setColor(0xFFA0A0A0);
         for (int y = 0; y < 80; y += 16) {
           for (int x = 0; x < 80; x += 16) {
             canvas.save();
@@ skipping 165 matching lines @@
 private:
     SkBitmap fBitmap, fCheckerboard;
     typedef GM INHERITED;
 };
 
 //////////////////////////////////////////////////////////////////////////////
 
 DEF_GM( return new XfermodeImageFilterGM; );
 
 }