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

gm/shadertext3.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 "SkGradientShader.h"
 #include "SkUnitMappers.h"
 
 namespace skiagm {
 
 static void makebm(SkBitmap* bm, SkBitmap::Config config, int w, int h) {
-    bm->setConfig(config, w, h);
-    bm->allocPixels();
+    bm->allocConfigPixels(config, w, h);
     bm->eraseColor(SK_ColorTRANSPARENT);
 
     SkCanvas    canvas(*bm);
     SkScalar    s = SkIntToScalar(SkMin32(w, h));
     static const SkPoint     kPts0[] = { { 0, 0 }, { s, s } };
     static const SkPoint     kPts1[] = { { s/2, 0 }, { s/2, s } };
     static const SkScalar    kPos[] = { 0, SK_Scalar1/2, SK_Scalar1 };
     static const SkColor kColors0[] = {0x80F00080, 0xF0F08000, 0x800080F0 };
     static const SkColor kColors1[] = {0xF08000F0, 0x8080F000, 0xF000F080 };
 
@@ skipping 108 matching lines @@
     typedef GM INHERITED;
 };
 
 ///////////////////////////////////////////////////////////////////////////////
 
 #ifndef SK_BUILD_FOR_ANDROID
 static GM* MyFactory(void*) { return new ShaderText3GM; }
 static GMRegistry reg(MyFactory);
 #endif
 }