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

gm/tileimagefilter.cpp

Index: gm/tileimagefilter.cpp
diff --git a/gm/tileimagefilter.cpp b/gm/tileimagefilter.cpp
index 2d6160fe8212e506a0f7054702d4afb4b70b6e78..d0acad7e69aaffba293439be1bdbfbb2c14e6bb2 100644
--- a/gm/tileimagefilter.cpp
+++ b/gm/tileimagefilter.cpp
@@ -27,8 +27,7 @@ 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(0xFF000000);
@@ -41,8 +40,7 @@ protected:
     }
 
     void make_checkerboard() {
-        fCheckerboard.setConfig(SkBitmap::kARGB_8888_Config, 80, 80);
-        fCheckerboard.allocPixels();
+        fCheckerboard.allocN32Pixels(80, 80);
         SkBitmapDevice device(fCheckerboard);
         SkCanvas canvas(&device);
         canvas.clear(0x00000000);