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

gm/bitmapscroll.cpp

Index: gm/bitmapscroll.cpp
diff --git a/gm/bitmapscroll.cpp b/gm/bitmapscroll.cpp
index a1c0ce4702dcc760ba04b63a6fd641633900a58f..b14b0475dd6242a0ece52737afeb7a943be5a481 100644
--- a/gm/bitmapscroll.cpp
+++ b/gm/bitmapscroll.cpp
@@ -26,10 +26,7 @@ static void make_bitmap(int quarterWidth, int quarterHeight, SkBitmap *bitmap) {
     pAlphaGray.setColor(0x66888888);
 
     // Prepare bitmap, and a canvas that draws into it.
-    bitmap->reset();
-    bitmap->setConfig(SkBitmap::kARGB_8888_Config,
-                      quarterWidth*4, quarterHeight*4);
-    bitmap->allocPixels();
+    bitmap->allocN32Pixels(quarterWidth*4, quarterHeight*4);
     SkCanvas canvas(*bitmap);
 
     SkScalar w = SkIntToScalar(quarterWidth);