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

gm/displacement.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 "SkDisplacementMapEffect.h"
 #include "SkBitmapSource.h"
@@ skipping 14 matching lines @@
     void make_bitmaps() {
         make_bitmap();
         make_checkerboard(&fCheckerboard, 80, 80);
         make_checkerboard(&fSmall,        64, 64);
         make_checkerboard(&fLarge,        96, 96);
         make_checkerboard(&fLargeW,       96, 64);
         make_checkerboard(&fLargeH,       64, 96);
     }
 
     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(0xFF884422);
         paint.setTextSize(SkIntToScalar(96));
         const char* str = "g";
         canvas.drawText(str, strlen(str), SkIntToScalar(15), SkIntToScalar(55), paint);
     }
 
     void make_checkerboard(SkBitmap* bitmap, int w, int h) {
-        bitmap->setConfig(SkBitmap::kARGB_8888_Config, w, h);
-        bitmap->allocPixels();
+        bitmap->allocN32Pixels(w, h);
         SkBitmapDevice device(*bitmap);
         SkCanvas canvas(&device);
         canvas.clear(0x00000000);
         SkPaint darkPaint;
         darkPaint.setColor(0xFF804020);
         SkPaint lightPaint;
         lightPaint.setColor(0xFF244484);
         for (int y = 0; y < h; y += 16) {
           for (int x = 0; x < w; x += 16) {
             canvas.save();
@@ skipping 163 matching lines @@
     SkBitmap fBitmap, fCheckerboard, fSmall, fLarge, fLargeW, fLargeH;
     bool fInitialized;
 };
 
 //////////////////////////////////////////////////////////////////////////////
 
 static GM* MyFactory(void*) { return new DisplacementMapGM; }
 static GMRegistry reg(MyFactory);
 
 }