store SkAlphaType inside SkBitmap, on road to support unpremul

bench/BitmapBench.cpp

Index: bench/BitmapBench.cpp
diff --git a/bench/BitmapBench.cpp b/bench/BitmapBench.cpp
index 982c917c02691319bc9a4b911c625b88d0f50066..6dc759283e4952b03b274e47e89ae3bc1d1a381e 100644
--- a/bench/BitmapBench.cpp
+++ b/bench/BitmapBench.cpp
@@ -126,7 +126,7 @@ protected:
             fBitmap = bm;
         }
 
-        fBitmap.setIsOpaque(fIsOpaque);
+        fBitmap.setAlphaType(fIsOpaque ? kOpaque_SkAlphaType : kPremul_SkAlphaType);
         fBitmap.setIsVolatile(fIsVolatile);
     }