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/bleed.cpp b/gm/bleed.cpp
index 8817604..c343d84 100644
--- a/gm/bleed.cpp
+++ b/gm/bleed.cpp
@@ -24,10 +24,7 @@
     static const SkPMColor kBlack = SkPreMultiplyColor(SK_ColorBLACK);
     static const SkPMColor kWhite = SkPreMultiplyColor(SK_ColorWHITE);
 
-    result->setConfig(SkBitmap::kARGB_8888_Config, width, height, 0,
-                      kOpaque_SkAlphaType);
-    result->allocPixels();
-    SkAutoLockPixels lock(*result);
+    result->allocN32Pixels(width, height, true);
 
     SkPMColor* scanline = result->getAddr32(0, 0);
     for (int x = 0; x < width; ++x) {