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/gmmain.cpp b/gm/gmmain.cpp
index 86d96fe..e7f2365 100644
--- a/gm/gmmain.cpp
+++ b/gm/gmmain.cpp
@@ -475,8 +475,7 @@
      */
     static void setup_bitmap(const ConfigData& gRec, SkISize& size,
                              SkBitmap* bitmap) {
-        bitmap->setConfig(gRec.fConfig, size.width(), size.height());
-        bitmap->allocPixels();
+        bitmap->allocConfigPixels(gRec.fConfig, size.width(), size.height());
         bitmap->eraseColor(SK_ColorTRANSPARENT);
     }