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/xfermodes3.cpp b/gm/xfermodes3.cpp
index baac3cd..d01c019 100644
--- a/gm/xfermodes3.cpp
+++ b/gm/xfermodes3.cpp
@@ -188,11 +188,10 @@
             SkPackARGB32(0xFF, 0x40, 0x40, 0x40)
         };
         SkBitmap bg;
-        bg.setConfig(SkBitmap::kARGB_8888_Config, 2, 2);
+        bg.setConfig(SkBitmap::kARGB_8888_Config, 2, 2, 0, kOpaque_SkAlphaType);
         bg.allocPixels();
         SkAutoLockPixels bgAlp(bg);
         memcpy(bg.getPixels(), kCheckData, sizeof(kCheckData));
-        bg.setIsOpaque(true);
 
         fBGShader.reset(SkShader::CreateBitmapShader(bg,
                                                      SkShader::kRepeat_TileMode,