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/giantbitmap.cpp b/gm/giantbitmap.cpp
index ab75dc7..c284809 100644
--- a/gm/giantbitmap.cpp
+++ b/gm/giantbitmap.cpp
@@ -27,8 +27,7 @@
     const SkBitmap& getBitmap() {
         if (NULL == fBM) {
             fBM = new SkBitmap;
-            fBM->setConfig(SkBitmap::kARGB_8888_Config, W, H);
-            fBM->allocPixels();
+            fBM->allocN32Pixels(W, H);
             fBM->eraseColor(SK_ColorWHITE);
 
             const SkColor colors[] = {