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

gm/gmmain.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.
  */
 
 /*
  * Code for the "gm" (Golden Master) rendering comparison tool.
  *
@@ skipping 457 matching lines @@
      * After you've rendered the GM into the SkBitmap, you must call
      * complete_bitmap()!
      *
      * @todo thudson 22 April 2011 - could refactor this to take in
      * a factory to generate the context, always call readPixels()
      * (logically a noop for rasters, if wasted time), and thus collapse the
      * GPU special case and also let this be used for SkPicture testing.
      */
     static void setup_bitmap(const ConfigData& gRec, SkISize& size,
                              SkBitmap* bitmap) {
-        bitmap->setConfig(gRec.fConfig, size.width(), size.height());
-        bitmap->allocPixels();
+        bitmap->allocConfigPixels(gRec.fConfig, size.width(), size.height());
         bitmap->eraseColor(SK_ColorTRANSPARENT);
     }
 
     /**
      * Any finalization steps we need to perform on the SkBitmap after
      * we have rendered the GM into it.
      *
      * It's too bad that we are throwing away alpha channel data
      * we could otherwise be examining, but this had always been happening
      * before... it was buried within the compare() method at
@@ skipping 1922 matching lines @@
     if (FLAGS_forceBWtext) {
         canvas->setDrawFilter(SkNEW(BWTextDrawFilter))->unref();
     }
 }
 
 #if !defined(SK_BUILD_FOR_IOS) && !defined(SK_BUILD_FOR_NACL)
 int main(int argc, char * const argv[]) {
     return tool_main(argc, (char**) argv);
 }
 #endif