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

gm/arithmode.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 "SkCanvas.h"
 #include "SkColorPriv.h"
 #include "SkShader.h"
 
 #include "SkArithmeticMode.h"
 #include "SkGradientShader.h"
 #define WW  100
 #define HH  32
 
 static SkBitmap make_bm() {
     SkBitmap bm;
-    bm.setConfig(SkBitmap::kARGB_8888_Config, WW, HH);
-    bm.allocPixels();
+    bm.allocN32Pixels(WW, HH);
     bm.eraseColor(SK_ColorTRANSPARENT);
     return bm;
 }
 
 static SkBitmap make_src() {
     SkBitmap bm = make_bm();
     SkCanvas canvas(bm);
     SkPaint paint;
     SkPoint pts[] = { {0, 0}, {SkIntToScalar(WW), SkIntToScalar(HH)} };
     SkColor colors[] = {
@@ skipping 92 matching lines @@
     }
 
 private:
     typedef GM INHERITED;
 };
 
 ///////////////////////////////////////////////////////////////////////////////
 
 static skiagm::GM* MyFactory(void*) { return new ArithmodeGM; }
 static skiagm::GMRegistry reg(MyFactory);