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

gm/imagefilterscropped.cpp

Index: gm/imagefilterscropped.cpp
diff --git a/gm/imagefilterscropped.cpp b/gm/imagefilterscropped.cpp
index 8f023f7208e209b22403c4642f4cc8b207a6340d..9f2d70025bd75807cfa4462d8cda05ef684bf825 100644
--- a/gm/imagefilterscropped.cpp
+++ b/gm/imagefilterscropped.cpp
@@ -60,8 +60,7 @@ static void draw_bitmap(SkCanvas* canvas, const SkRect& r, SkImageFilter* imf) {
     r.roundOut(&bounds);
 
     SkBitmap bm;
-    bm.setConfig(SkBitmap::kARGB_8888_Config, bounds.width(), bounds.height());
-    bm.allocPixels();
+    bm.allocN32Pixels(bounds.width(), bounds.height());
     bm.eraseColor(SK_ColorTRANSPARENT);
     SkCanvas c(bm);
     draw_path(&c, r, NULL);
@@ -80,8 +79,7 @@ static void draw_sprite(SkCanvas* canvas, const SkRect& r, SkImageFilter* imf) {
     r.roundOut(&bounds);
 
     SkBitmap bm;
-    bm.setConfig(SkBitmap::kARGB_8888_Config, bounds.width(), bounds.height());
-    bm.allocPixels();
+    bm.allocN32Pixels(bounds.width(), bounds.height());
     bm.eraseColor(SK_ColorRED);
     SkCanvas c(bm);