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/bench/BitmapBench.cpp b/bench/BitmapBench.cpp
index 982c917..6dc7592 100644
--- a/bench/BitmapBench.cpp
+++ b/bench/BitmapBench.cpp
@@ -126,7 +126,7 @@
             fBitmap = bm;
         }
 
-        fBitmap.setIsOpaque(fIsOpaque);
+        fBitmap.setAlphaType(fIsOpaque ? kOpaque_SkAlphaType : kPremul_SkAlphaType);
         fBitmap.setIsVolatile(fIsVolatile);
     }