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/optimizations.cpp b/gm/optimizations.cpp
index 8d0b7ca..a404ad7 100644
--- a/gm/optimizations.cpp
+++ b/gm/optimizations.cpp
@@ -395,10 +395,7 @@
         static const unsigned int kCheckerboardWidth = 16;
         static const unsigned int kCheckerboardHeight = 16;
 
-        fCheckerboard.setConfig(SkBitmap::kARGB_8888_Config,
-                                kCheckerboardWidth, kCheckerboardHeight);
-        fCheckerboard.allocPixels();
-        SkAutoLockPixels lock(fCheckerboard);
+        fCheckerboard.allocN32Pixels(kCheckerboardWidth, kCheckerboardHeight);
         for (unsigned int y = 0; y < kCheckerboardHeight; y += 2) {
             SkPMColor* scanline = fCheckerboard.getAddr32(0, y);
             for (unsigned int x = 0; x < kCheckerboardWidth; x += 2) {