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 9b3a6a29dd082cf236a4259c8c743312e27b947e..6f9121947c0cbf6f4e2e34aa13716f48b4cddfe7 100644
--- a/bench/BitmapBench.cpp
+++ b/bench/BitmapBench.cpp
@@ -128,7 +128,7 @@ protected:
         if (fBitmap.getColorTable()) {
             fBitmap.getColorTable()->setIsOpaque(fIsOpaque);
         }
-        fBitmap.setIsOpaque(fIsOpaque);
+        fBitmap.setAlphaType(fIsOpaque ? kOpaque_SkAlphaType : kPremul_SkAlphaType);
         fBitmap.setIsVolatile(fIsVolatile);
     }