add legacy/helper allocN32Pixels, and convert gm to use it

gm/optimizations.cpp

Index: gm/optimizations.cpp
diff --git a/gm/optimizations.cpp b/gm/optimizations.cpp
index 8d0b7caa84b500423cc8354cfc13494136787b31..a404ad7c55c9f023f28d8b94546aa228075b8579 100644
--- a/gm/optimizations.cpp
+++ b/gm/optimizations.cpp
@@ -395,10 +395,7 @@ private:
         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) {