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/pathopsinverse.cpp b/gm/pathopsinverse.cpp
index 2fbc8f7..c1ffd94 100644
--- a/gm/pathopsinverse.cpp
+++ b/gm/pathopsinverse.cpp
@@ -36,8 +36,7 @@
 
     SkColor blend(SkColor one, SkColor two) {
         SkBitmap temp;
-        temp.setConfig(SkBitmap::kARGB_8888_Config, 1, 1);
-        temp.allocPixels();
+        temp.allocN32Pixels(1, 1);
         SkCanvas canvas(temp);
         canvas.drawColor(one);
         canvas.drawColor(two);