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/xfermodes.cpp b/gm/xfermodes.cpp
index 7ee1e80..353bebf 100644
--- a/gm/xfermodes.cpp
+++ b/gm/xfermodes.cpp
@@ -14,8 +14,7 @@
 
 static void make_bitmaps(int w, int h, SkBitmap* src, SkBitmap* dst,
                          SkBitmap* transparent) {
-    src->setConfig(SkBitmap::kARGB_8888_Config, w, h);
-    src->allocPixels();
+    src->allocN32Pixels(w, h);
     src->eraseColor(SK_ColorTRANSPARENT);
 
     SkPaint p;