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

gm/hairmodes.cpp

Index: gm/hairmodes.cpp
diff --git a/gm/hairmodes.cpp b/gm/hairmodes.cpp
index 6fd72fcf55afc67e62f5ebfebd4572979c6656c8..4aa999d3838f4c2551200c418c3b2270dfc6495c 100644
--- a/gm/hairmodes.cpp
+++ b/gm/hairmodes.cpp
@@ -60,8 +60,7 @@ static SkScalar drawCell(SkCanvas* canvas, SkXfermode* mode, SkAlpha a0, SkAlpha
 
 static SkShader* make_bg_shader() {
     SkBitmap bm;
-    bm.setConfig(SkBitmap::kARGB_8888_Config, 2, 2);
-    bm.allocPixels();
+    bm.allocN32Pixels(2, 2);
     *bm.getAddr32(0, 0) = *bm.getAddr32(1, 1) = 0xFFFFFFFF;
     *bm.getAddr32(1, 0) = *bm.getAddr32(0, 1) = SkPackARGB32(0xFF, 0xCC, 0xCC, 0xCC);