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

This is an intermediate api, but might help us quickly get to a point where
no one is creating bitmaps in a 2-step process (setConfig + alloc).

BUG=skia:
R=halcanary@google.com

Review URL: https://codereview.chromium.org/140593005

git-svn-id: http://skia.googlecode.com/svn/trunk@13182 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gm/spritebitmap.cpp b/gm/spritebitmap.cpp
index 8aaa6ea..5884efe 100644
--- a/gm/spritebitmap.cpp
+++ b/gm/spritebitmap.cpp
@@ -10,8 +10,7 @@
 #include "SkBlurImageFilter.h"
 
 static void make_bm(SkBitmap* bm) {
-    bm->setConfig(SkBitmap::kARGB_8888_Config, 100, 100);
-    bm->allocPixels();
+    bm->allocN32Pixels(100, 100);
     bm->eraseColor(SK_ColorBLUE);
 
     SkCanvas canvas(*bm);