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

This is an intermediate api, but might help us quickly get to a point where
no one is creating bitmaps in a 2-step process (setConfig + alloc).

BUG=skia:
R=halcanary@google.com

Review URL: https://codereview.chromium.org/140593005

git-svn-id: http://skia.googlecode.com/svn/trunk@13182 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gm/imagefiltersbase.cpp b/gm/imagefiltersbase.cpp
index 5a0b0b6..528dd19 100644
--- a/gm/imagefiltersbase.cpp
+++ b/gm/imagefiltersbase.cpp
@@ -119,8 +119,7 @@
     r.roundOut(&bounds);
 
     SkBitmap bm;
-    bm.setConfig(SkBitmap::kARGB_8888_Config, bounds.width(), bounds.height());
-    bm.allocPixels();
+    bm.allocN32Pixels(bounds.width(), bounds.height());
     bm.eraseColor(SK_ColorTRANSPARENT);
     SkCanvas c(bm);
     draw_path(&c, r, NULL);
@@ -136,8 +135,7 @@
     r.roundOut(&bounds);
 
     SkBitmap bm;
-    bm.setConfig(SkBitmap::kARGB_8888_Config, bounds.width(), bounds.height());
-    bm.allocPixels();
+    bm.allocN32Pixels(bounds.width(), bounds.height());
     bm.eraseColor(SK_ColorTRANSPARENT);
     SkCanvas c(bm);
     draw_path(&c, r, NULL);