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/bitmapshader.cpp b/gm/bitmapshader.cpp
index 4fd68ee..d1915c9 100644
--- a/gm/bitmapshader.cpp
+++ b/gm/bitmapshader.cpp
@@ -17,8 +17,7 @@
     SkPaint bluePaint;
     bluePaint.setColor(SK_ColorBLUE);
 
-    bm->setConfig(SkBitmap::kARGB_8888_Config, 20, 20);
-    bm->allocPixels();
+    bm->allocN32Pixels(20, 20);
     bm->eraseColor(SK_ColorRED);
 
     SkCanvas canvas(*bm);
@@ -29,8 +28,7 @@
     SkPaint circlePaint;
     circlePaint.setColor(SK_ColorBLACK);
 
-    bm->setConfig(SkBitmap::kA8_Config, 20, 20);
-    bm->allocPixels();
+    bm->allocConfigPixels(SkBitmap::kA8_Config, 20, 20);
     bm->eraseColor(SK_ColorTRANSPARENT);
 
     SkCanvas canvas(*bm);