replace setConfig+allocPixels with single call

BUG=skia:

Review URL: https://codereview.chromium.org/162643002

git-svn-id: http://skia.googlecode.com/svn/trunk@13426 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/tests/MipMapTest.cpp b/tests/MipMapTest.cpp
index aca6973..33f4672 100644
--- a/tests/MipMapTest.cpp
+++ b/tests/MipMapTest.cpp
@@ -15,8 +15,7 @@
     // should fix that to support 1 X N, where N > 1 to return non-null.
     int w = 2 + rand.nextU() % 1000;
     int h = 2 + rand.nextU() % 1000;
-    bm->setConfig(SkBitmap::kARGB_8888_Config, w, h);
-    bm->allocPixels();
+    bm->allocN32Pixels(w, h);
     bm->eraseColor(SK_ColorWHITE);
 }