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

gm/bicubicfilter.cpp

Index: gm/bicubicfilter.cpp
diff --git a/gm/bicubicfilter.cpp b/gm/bicubicfilter.cpp
index 7eb0aeebb276e5db417fc6cb0fa7f7ef0c1259db..825175e252752ee8584286fdf1f0983635cdbe75 100644
--- a/gm/bicubicfilter.cpp
+++ b/gm/bicubicfilter.cpp
@@ -25,9 +25,7 @@ protected:
     void make_checkerboard(int width, int height) {
         SkASSERT(width % 2 == 0);
         SkASSERT(height % 2 == 0);
-        fCheckerboard.setConfig(SkBitmap::kARGB_8888_Config, width, height);
-        fCheckerboard.allocPixels();
-        SkAutoLockPixels lock(fCheckerboard);
+        fCheckerboard.allocN32Pixels(width, height);
         for (int y = 0; y < height; y += 2) {
             SkPMColor* s = fCheckerboard.getAddr32(0, y);
             for (int x = 0; x < width; x += 2) {