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

gm/xfermodes3.cpp

Index: gm/xfermodes3.cpp
diff --git a/gm/xfermodes3.cpp b/gm/xfermodes3.cpp
index d01c0190d4a4391f6f99a1a8052b493a47acdc57..bb7d6149c5e3c790a2720b65b4b13597c1832bd2 100644
--- a/gm/xfermodes3.cpp
+++ b/gm/xfermodes3.cpp
@@ -188,8 +188,7 @@ private:
             SkPackARGB32(0xFF, 0x40, 0x40, 0x40)
         };
         SkBitmap bg;
-        bg.setConfig(SkBitmap::kARGB_8888_Config, 2, 2, 0, kOpaque_SkAlphaType);
-        bg.allocPixels();
+        bg.allocN32Pixels(2, 2, true);
         SkAutoLockPixels bgAlp(bg);
         memcpy(bg.getPixels(), kCheckData, sizeof(kCheckData));
 
@@ -212,8 +211,7 @@ private:
                                                           SkShader::kRepeat_TileMode))->unref();
 
         SkBitmap bmp;
-        bmp.setConfig(SkBitmap::kARGB_8888_Config, kSize, kSize);
-        bmp.allocPixels();
+        bmp.allocN32Pixels(kSize, kSize);
         SkCanvas bmpCanvas(bmp);
 
         bmpCanvas.clear(SK_ColorTRANSPARENT);