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/modecolorfilters.cpp b/gm/modecolorfilters.cpp
index 9859faa..9ac8ddd 100644
--- a/gm/modecolorfilters.cpp
+++ b/gm/modecolorfilters.cpp
@@ -38,8 +38,7 @@
 // draws a background behind each test rect to see transparency
 static SkShader* make_bg_shader(int checkSize) {
     SkBitmap bmp;
-    bmp.setConfig(SkBitmap::kARGB_8888_Config, 2 * checkSize, 2 * checkSize);
-    bmp.allocPixels();
+    bmp.allocN32Pixels(2 * checkSize, 2 * checkSize);
     SkCanvas canvas(bmp);
     canvas.clear(0xFF800000);
     SkPaint paint;