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/BitmapRectBench.cpp b/bench/BitmapRectBench.cpp
index 1dd98ef..104cb24 100644
--- a/bench/BitmapRectBench.cpp
+++ b/bench/BitmapRectBench.cpp
@@ -66,7 +66,7 @@
 
     virtual void onPreDraw() SK_OVERRIDE {
         fBitmap.allocPixels();
-        fBitmap.setIsOpaque(true);
+        fBitmap.setAlphaType(kOpaque_SkAlphaType);
         fBitmap.eraseColor(SK_ColorBLACK);
         draw_into_bitmap(fBitmap);