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

gm/imagefiltersgraph.cpp

 /*
  * Copyright 2012 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"
@@ skipping 73 matching lines @@
         SK_DECLARE_STATIC_ONCE(once);
         SkOnce(&once, init_flattenable, &dummy);
     }
 
 protected:
     virtual SkString onShortName() {
         return SkString("imagefiltersgraph");
     }
 
     void make_bitmap() {
-        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 100 matching lines @@
 private:
     typedef GM INHERITED;
     SkBitmap fBitmap;
     bool fInitialized;
 };
 
 ///////////////////////////////////////////////////////////////////////////////
 
 static skiagm::GM* MyFactory(void*) { return new ImageFiltersGraphGM; }
 static skiagm::GMRegistry reg(MyFactory);