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/displacement.cpp b/gm/displacement.cpp
index cc66401..ef6207a 100644
--- a/gm/displacement.cpp
+++ b/gm/displacement.cpp
@@ -32,8 +32,7 @@
     }
 
     void make_bitmap() {
-        fBitmap.setConfig(SkBitmap::kARGB_8888_Config, 80, 80);
-        fBitmap.allocPixels();
+        fBitmap.allocN32Pixels(80, 80);
         SkBitmapDevice device(fBitmap);
         SkCanvas canvas(&device);
         canvas.clear(0x00000000);
@@ -46,8 +45,7 @@
     }
 
     void make_checkerboard(SkBitmap* bitmap, int w, int h) {
-        bitmap->setConfig(SkBitmap::kARGB_8888_Config, w, h);
-        bitmap->allocPixels();
+        bitmap->allocN32Pixels(w, h);
         SkBitmapDevice device(*bitmap);
         SkCanvas canvas(&device);
         canvas.clear(0x00000000);