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

gm/imagefiltersbase.cpp

Index: gm/imagefiltersbase.cpp
diff --git a/gm/imagefiltersbase.cpp b/gm/imagefiltersbase.cpp
index 5a0b0b6a87748bf4cda650e760b35d3f429bed35..528dd19f33af71c356d842f986628695e1a1cf5e 100644
--- a/gm/imagefiltersbase.cpp
+++ b/gm/imagefiltersbase.cpp
@@ -119,8 +119,7 @@ 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);
@@ -136,8 +135,7 @@ 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_ColorTRANSPARENT);
     SkCanvas c(bm);
     draw_path(&c, r, NULL);