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

gm/xfermodes.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 "SkBitmap.h"
 #include "SkShader.h"
 #include "SkXfermode.h"
 
 namespace skiagm {
 
 static void make_bitmaps(int w, int h, SkBitmap* src, SkBitmap* dst,
                          SkBitmap* transparent) {
-    src->setConfig(SkBitmap::kARGB_8888_Config, w, h);
-    src->allocPixels();
+    src->allocN32Pixels(w, h);
     src->eraseColor(SK_ColorTRANSPARENT);
 
     SkPaint p;
     p.setAntiAlias(true);
 
     SkRect r;
     SkScalar ww = SkIntToScalar(w);
     SkScalar hh = SkIntToScalar(h);
 
     {
@@ skipping 256 matching lines @@
 private:
     typedef GM INHERITED;
 };
 
 //////////////////////////////////////////////////////////////////////////////
 
 static GM* MyFactory(void*) { return new XfermodesGM; }
 static GMRegistry reg(MyFactory);
 
 }