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

gm/tablecolorfilter.cpp

Index: gm/tablecolorfilter.cpp
diff --git a/gm/tablecolorfilter.cpp b/gm/tablecolorfilter.cpp
index 49e342d0022e358413f7564bca23467d1dbb5662..f806caad6ae850e68f2a521c7b2dca0bc4730ffb 100644
--- a/gm/tablecolorfilter.cpp
+++ b/gm/tablecolorfilter.cpp
@@ -13,8 +13,7 @@
 static void make_bm0(SkBitmap* bm) {
     int W = 120;
     int H = 120;
-    bm->setConfig(SkBitmap::kARGB_8888_Config, W, H);
-    bm->allocPixels();
+    bm->allocN32Pixels(W, H);
     bm->eraseColor(SK_ColorTRANSPARENT);
 
     SkCanvas canvas(*bm);
@@ -32,8 +31,7 @@ static void make_bm0(SkBitmap* bm) {
 static void make_bm1(SkBitmap* bm) {
     int W = 120;
     int H = 120;
-    bm->setConfig(SkBitmap::kARGB_8888_Config, W, H);
-    bm->allocPixels();
+    bm->allocN32Pixels(W, H);
     bm->eraseColor(SK_ColorTRANSPARENT);
 
     SkCanvas canvas(*bm);