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

gm/bitmapsource.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 "SkBitmapSource.h"
 
 // This GM exercises the SkBitmapSource ImageFilter class.
 
 class BitmapSourceGM : public skiagm::GM {
 public:
     BitmapSourceGM() {
     }
 
 protected:
     virtual SkString onShortName() SK_OVERRIDE {
         return SkString("bitmapsource");
     }
 
     void makeBitmap() {
-        fBitmap.setConfig(SkBitmap::kARGB_8888_Config, 100, 100);
-        fBitmap.allocPixels();
+        fBitmap.allocN32Pixels(100, 100);
         SkBitmapDevice device(fBitmap);
         SkCanvas canvas(&device);
         canvas.clear(0x00000000);
         SkPaint paint;
         paint.setAntiAlias(true);
         paint.setColor(0xFFFFFFFF);
         paint.setTextSize(SkIntToScalar(96));
         const char* str = "e";
         canvas.drawText(str, strlen(str), SkIntToScalar(20), SkIntToScalar(70), paint);
     }
@@ skipping 45 matching lines @@
     }
 
 private:
     SkBitmap fBitmap;
     typedef GM INHERITED;
 };
 
 ///////////////////////////////////////////////////////////////////////////////
 
 DEF_GM( return new BitmapSourceGM; )