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/extractbitmap.cpp b/gm/extractbitmap.cpp
index 5206741..efc5d35 100644
--- a/gm/extractbitmap.cpp
+++ b/gm/extractbitmap.cpp
@@ -16,8 +16,7 @@
 static void create_bitmap(SkBitmap* bitmap) {
     const int W = 100;
     const int H = 100;
-    bitmap->setConfig(SkBitmap::kARGB_8888_Config, W, H);
-    bitmap->allocPixels();
+    bitmap->allocN32Pixels(W, H);
 
     SkCanvas canvas(*bitmap);
     canvas.drawColor(SK_ColorRED);