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/clippedbitmapshaders.cpp b/gm/clippedbitmapshaders.cpp
index 4087c89..72c0474 100644
--- a/gm/clippedbitmapshaders.cpp
+++ b/gm/clippedbitmapshaders.cpp
@@ -27,15 +27,12 @@
 
 static SkBitmap create_bitmap() {
     SkBitmap bmp;
-    bmp.setConfig(SkBitmap::kARGB_8888_Config, 2, 2);
-    bmp.allocPixels();
-    bmp.lockPixels();
+    bmp.allocN32Pixels(2, 2);
     uint32_t* pixels = reinterpret_cast<uint32_t*>(bmp.getPixels());
     pixels[0] = SkPreMultiplyColor(SK_ColorRED);
     pixels[1] = SkPreMultiplyColor(SK_ColorGREEN);
     pixels[2] = SkPreMultiplyColor(SK_ColorBLACK);
     pixels[3] = SkPreMultiplyColor(SK_ColorBLUE);
-    bmp.unlockPixels();
 
     return bmp;
 }