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

gm/displacement.cpp

Index: gm/displacement.cpp
diff --git a/gm/displacement.cpp b/gm/displacement.cpp
index cc664012da6e3612baec272d123d33259bb8bc39..ef6207a20d293584824a271f41d419738c007d4d 100644
--- a/gm/displacement.cpp
+++ b/gm/displacement.cpp
@@ -32,8 +32,7 @@ protected:
     }
 
     void make_bitmap() {
-        fBitmap.setConfig(SkBitmap::kARGB_8888_Config, 80, 80);
-        fBitmap.allocPixels();
+        fBitmap.allocN32Pixels(80, 80);
         SkBitmapDevice device(fBitmap);
         SkCanvas canvas(&device);
         canvas.clear(0x00000000);
@@ -46,8 +45,7 @@ protected:
     }
 
     void make_checkerboard(SkBitmap* bitmap, int w, int h) {
-        bitmap->setConfig(SkBitmap::kARGB_8888_Config, w, h);
-        bitmap->allocPixels();
+        bitmap->allocN32Pixels(w, h);
         SkBitmapDevice device(*bitmap);
         SkCanvas canvas(&device);
         canvas.clear(0x00000000);