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

gm/xfermodeimagefilter.cpp

Index: gm/xfermodeimagefilter.cpp
diff --git a/gm/xfermodeimagefilter.cpp b/gm/xfermodeimagefilter.cpp
index 936af46ec6604a7fcc76459f146f747649f8bde2..f843d40dae3e7287fb8bcf135bc13dfffbffeb5f 100644
--- a/gm/xfermodeimagefilter.cpp
+++ b/gm/xfermodeimagefilter.cpp
@@ -29,8 +29,7 @@ protected:
     }
 
     void make_bitmap() {
-        fBitmap.setConfig(SkBitmap::kARGB_8888_Config, 80, 80);
-        fBitmap.allocPixels();
+        fBitmap.allocN32Pixels(80, 80);
         SkBitmapDevice device(fBitmap);
         SkCanvas canvas(&device);
         canvas.clear(0x00000000);
@@ -43,8 +42,7 @@ protected:
     }
 
     void make_checkerboard() {
-        fCheckerboard.setConfig(SkBitmap::kARGB_8888_Config, 80, 80);
-        fCheckerboard.allocPixels();
+        fCheckerboard.allocN32Pixels(80, 80);
         SkBitmapDevice device(fCheckerboard);
         SkCanvas canvas(&device);
         canvas.clear(0x00000000);