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

gm/dropshadowimagefilter.cpp

Index: gm/dropshadowimagefilter.cpp
diff --git a/gm/dropshadowimagefilter.cpp b/gm/dropshadowimagefilter.cpp
index 6cffd5bb4a006a3ff809946f69d8533f371503fc..e9ed583a395d9fd5b9b09ac58ee0af91bbce97c5 100644
--- a/gm/dropshadowimagefilter.cpp
+++ b/gm/dropshadowimagefilter.cpp
@@ -54,8 +54,7 @@ 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);
@@ -74,8 +73,7 @@ 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);