store SkAlphaType inside SkBitmap, on road to support unpremul

BUG=
R=bsalomon@google.com, scroggo@google.com

Review URL: https://codereview.chromium.org/25275004

git-svn-id: http://skia.googlecode.com/svn/trunk@11877 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gm/xfermodes.cpp b/gm/xfermodes.cpp
index 65f53bf..e839b0e 100644
--- a/gm/xfermodes.cpp
+++ b/gm/xfermodes.cpp
@@ -60,9 +60,8 @@
     }
 
     virtual void onOnceBeforeDraw() SK_OVERRIDE {
-        fBG.setConfig(SkBitmap::kARGB_4444_Config, 2, 2, 4);
+        fBG.setConfig(SkBitmap::kARGB_4444_Config, 2, 2, 4, kOpaque_SkAlphaType);
         fBG.setPixels(gData);
-        fBG.setIsOpaque(true);
 
         make_bitmaps(W, H, &fSrcB, &fDstB);
     }